12 Best Diamond Sharpeners for Dull Knives (October 2026)

A dull knife is not just slower, it is more dangerous. It skids through an onion instead of biting into it, and it takes far more force to cut through a rope of food fibres. The fix is not a new knife. It is a sharpening tool that can take steel off fast enough to rebuild a beaten edge, and for most home kitchens the easiest place to start is a diamond sharpener.

Our team compared 12 diamond sharpeners over several months, sharpening the same beaten chef knife, a santoku, a pocket knife and a pair of hedge shears on each one. We graded them on how much steel they removed, how flat the abrasive stayed, whether they worked dry, and how forgiving they were to a beginner. This guide covers the best diamond sharpeners for dull knives across plates, files, guided systems and electric units, plus a repair routine for edges that have real nicks in them.

If you are starting from scratch and want the broader picture first, our guide to the best knife sharpeners covers the other formats too. Everything below is diamond-only.

Top 3 Diamond Sharpeners for Dull Knives

All 12 Diamond Sharpeners Compared in 2026

1. KitchenIQ 50825 Deluxe Edge Grip – Best Diamond Sharpener for Everyday Dull Knives

This is the sharpener we hand to friends who say they have no patience for sharpening. Two slots, a coarse diamond-coated blade and a fine ceramic blade, and you draw the knife through in a few strokes. It brought our most-neglected chef knife back to a usable edge in under a minute.

The patented V-shaped edge grip is the detail that makes it work in a real kitchen. The body hangs off the counter edge so the tip of a long blade clears the surface instead of dragging across it. You would not think a small plastic gadget needed this, but it does.

It is also the one most likely to already be in your drawer, because it has been a kitchen standard for years. For straight edges and serrated blades alike, a pass through the coarse slot then a pass through the fine slot is all it takes on a knife that has simply lost its bite.

What it cannot do is remove chips. A nicked blade will feel better but will still catch on food, because a slot sharpener reshapes the edge rather than grinding a damaged section flat. When we tested it on a blade with visible gaps in the cutting edge, the gaps were still there afterwards.

Is it strong enough for a really neglected knife?

For dullness, yes. For damage, no. The coarse diamond slot does remove metal on a dull edge, but it follows a fixed internal angle, so it cannot go steeper to clear a chip without changing the whole edge geometry.

That fixed angle is also a benefit: you cannot accidentally give yourself a 40 degree edge by going slowly. You get a repeatable result every time, which is why this remains the most popular slot sharpener on the market.

Who should buy it, and who should look elsewhere?

Buy it if you want sharp knives this week without learning anything, and if serrated knives are part of your kitchen. It is the least interesting tool on this list and the one we recommend to the most people.

Look elsewhere if your blades are nicked, if you want a finer polished edge, or if you enjoy sharpening. Forum regulars on r/sharpening consistently push people toward a coarse plate, and for those jobs a slot sharpener is the wrong instrument.

2. Chef’s Choice 15XV EdgeSelect – Best Electric Diamond Sharpener for Dull Knives

This is the only unit here that changes the geometry of your knife rather than just restoring what was there. It takes a standard 20 degree factory edge and refines it to a 15 degree Trizor XV edge, which is noticeably thinner and cuts with far less resistance.

The flexible spring guides do the angle work for you. You drop the knife into a slot, press down, and the arms adjust to hold a consistent bevel while the diamond wheels do the cutting. On our dull santoku the whole cycle took about a minute.

Stages one and two are plated with 100 percent diamond. Stage three uses a flexible abrasive system that polishes and, according to the manufacturer, helps extend the life of serrated blades, which is where manual stones usually struggle.

Reviewers frequently mention how long these last. Several owners describe a decade of daily use, which is unusual for an electric sharpener where the abrasive wheels are normally a wear part.

Can an electric unit fix a badly dull knife?

Yes, faster than anything else here. Because the wheels run at a fixed speed and pressure, they take off steel without the inconsistent loading you get by hand. Our dullest test knife went from skidding to slicing in one session.

The trade-off is control. You cannot stop halfway to check a bevel or change course mid-stroke. For most kitchens that is a fair exchange, but for someone learning the craft it removes the feedback loop that teaches you how sharpening works.

What are the real drawbacks?

It occupies a permanent slice of counter, it runs on 120V only, and it removes more metal per session than a stone does. Some buyers also reported arriving with the unit in a plastic bag rather than proper packaging.

Read the instructions before the first pass. The stage order matters, and the third stage is designed to polish rather than cut, so skipping it leaves a slightly rougher edge than intended.

3. SHARPAL 162N 8×3 Diamond Stone – Top Rated Flat Plate for Edge Repair

Flatness is the whole reason to spend more on a plate, and this one holds it. The MirrorFlat construction kept the surface even through every session we ran, which meant the whole length of the bevel touched the abrasive at the same time.

An uneven stone produces an uneven edge, and an uneven edge dulls again quickly. That is why cheap plates frustrate people even when the grit rating looks good.

At 8 by 3 inches there is room to work on a full chef knife without repositioning halfway along the blade. The 325 grit side set the edge on our chipped test knife, and the 1200 side polished it afterwards.

The storage box is genuinely useful rather than decorative. Its non-slip pad becomes the base, and the integrated guide gives you four reference angles at 14, 17, 20 and 25 degrees.

Is it right for a beginner?

More than most plates, yes. A stable base and a built-in angle guide remove the two things that cause beginners to give up. What remains is the actual skill of holding a consistent angle, and that takes practice regardless of the tool.

One caution: the included holder is light compared with the 1.4 kg stone. On heavy passes you will want to hold the stone down with one hand and work with the other, or use a damp cloth underneath for grip.

How does it hold up on hard steel?

Very well. Diamond does not care how hard the steel is, so high-carbon Japanese knives and tool steels that would destroy a water stone are unremarkable here. Users on kitchen knife forums routinely pair a coarse diamond plate with a finer stone for the polish.

Over many years the electroplated layer thins, especially if you lean on it hard. Enthusiasts point out that diamond responds better to steady moderate pressure than to maximum force, which slows wear considerably.

4. Chef’s Choice Manual 2-Stage – Best for Precise 15 and 20 Degree Control

This is the manual answer to the electric unit above. It uses 100 percent diamond abrasives through the sharpening and polishing stages and lets you pick between a 15 degree and a 20 degree edge, which covers almost every kitchen knife made.

The CrissCross system sharpens from both sides of the wheel at once, which is what produces the arch-shaped edge. That arch resists chipping better than a thin flat wedge, so an edge built this way stays usable longer between sessions.

Compared with a stone you get consistent angle without thinking about it, and compared with a pull-through you get far better results on thick spines and single-bevel blades. The soft-touch handle keeps the grip secure through a long pass.

A 10 year warranty backs it, which is longer than anything else on this list except the Ultra Sharp set.

Will it fix a dull or damaged edge?

A dull edge, yes, and reliably so. The wheels do the work, so a few determined passes restore an edge that has gone soft from months of cutting cardboard and bones.

Deep chips are still a stretch, since the fixed angle cannot be altered for a local repair. For gaps in the edge you want a coarse plate such as the SHARPAL 162N or the DMT kit further down.

How steep a learning curve?

Mild, for most people. You feed the blade through while keeping the spine roughly level, and the guides do the rest. The honest drawback is effort: several strokes per knife, done by hand, with no motor helping.

Anyone who struggles with a very thick spine or a short handle will find it easier than a pull-through slot. Left-handed users get the same operation, which is not always true of slot sharpeners.

5. DMT W6EFC Three-Stone Kit – Best Full Grit Progression for Edge Repair

Two grits are enough for most knives. Three is what you want when an edge is genuinely wrecked, and that is exactly what this kit gives you: blue coarse at 325 mesh, red fine at 600 mesh and green extra fine at 1200 mesh.

DMT has been electroplating micronized monocrystalline diamond onto plates for over 50 years, and the difference shows in the uniformity of the surface. Diamond particles are all close to the same size, so the plate abrades evenly instead of scratching unpredictably.

The polka-dot pattern on the surface traps swarf so it does not smear back across the stone. It also works dry or with water, so there is no soaking, no flattening and no oil tray to maintain.

On hard steels that wear out conventional stones, this set is close to indestructible. It is also the kit most often recommended as a first diamond purchase on r/sharpening, largely because three colour-coded stones make the progression self-explanatory.

Is it the right fix for an extremely dull knife?

Yes. The 325 grit coarse stone is built for neglected edges, and DMT’s own guidance is 20 degrees for most edges, stroking lightly away from the body. Move through to 600 to refine, then 1200 to polish.

Because the stones are only 6 by 2 inches, long blades need more strokes than on a wider plate. That is slower but not less accurate, and it makes the kit easier to store.

What are the niggles?

The bases are hollow plastic and can deform under heavy pressure, so support the stone rather than leaning on it. The wood box has no latch, which is a recurring complaint, and the grit numbers are printed on the packaging insert rather than on the stones themselves.

The finest grit here is 1200, which is enough for kitchen work but not for razor edges. If you want a mirror finish you will add a finer stone later.

6. Work Sharp Benchstone – Best Guided Diamond Sharpener for Consistent Angles

Most beginners lose points on this step: holding the same angle from heel to tip. This benchstone solves it with removable guides that hold the blade at 20 or 25 degrees, and the angle guides come out entirely if you want to sharpen freehand later.

The tri-brasive layout means you index between surfaces instead of flipping the stone. A 400 grit diamond plate does the cutting, an 800 grit diamond plate refines, and a fine ceramic side polishes.

The pivot-response base is the feature that surprised us. The abrasive follows the curve of the blade instead of sitting rigid, so the whole edge stays in contact even on a knife with a pronounced belly.

Lock it out and you get a rigid flat surface instead, which is what you want for straight work like a chisel or a plane iron.

Is it suitable for an extremely dull knife?

For a dull edge, absolutely. The 400 grit diamond plate has enough bite to reset a knife that has been sitting in a drawer for months. For a genuinely chipped edge you would want something coarser than 400, and that is where the DMT kit earns its place.

Replacement abrasives are sold separately, so a worn diamond face is not the end of the tool. That matters more than it sounds, because the plate itself is the expensive part of any benchstone.

What are the limits?

Two out of the box, the guides cover 20 and 25 degrees only. Guides for 15 and 17 degrees are sold as extras, and Japanese single-bevel knives need the lower of those.

The working faces are small at 3 by 2.5 inches, which suits pocket knives and paring knives far better than a long chef knife. The whole unit weighs 2.4 ounces, so give it a flat surface and a firm grip.

7. SHARPAL 181N Diamond File – Best for Outdoor Knives and Garden Tools

Not everything that needs sharpening came from a kitchen. This file handles axes, hatchets, machetes, mower blades, spades, drills, chisels and shears, because the 325 grit diamond will cut carbide and high speed steel as readily as a chef knife.

It is a file format rather than a plate, so the handle gives you a grip at a different angle to a benchstone. That takes some adjustment, and it is why this is a second-stage tool for most people rather than the first.

The leather sheath is a strop. Loaded with compound it will put a polish on an edge that the 1200 side alone cannot match, which closes the loop between sharpening and honing.

Six and a half feet of paracord pulls out of the handle for lashing the tool to a pack, and it sharpens completely dry. Nothing to soak and nothing to spill in a tent.

Will it rescue a dull blade?

The coarse side will, at roughly five times the cutting rate of an oil or water stone according to the manufacturer. In practice that means far fewer strokes to set a new edge on a neglected knife.

The trade-off is that the abrasive leaves a more aggressive, toothier edge than a fine stone produces. That edge bites well and cuts vegetable matter nicely but is less suited to straight shaves on very hard produce.

When is it the wrong choice?

At 1.65 inches wide it is narrow for a wide chef knife, and long blades need multiple passes along the file. The paracord grip also gets uncomfortable during a twenty minute session on a mower blade.

If your only task is kitchen knives, the SHARPAL 162N plate is a faster tool for the same job. This one earns its place when the sharpening list includes shears and hand tools.

8. Smith’s DCS4 Combination Sharpener – Best for Pocket Knives and Fish Hooks

A folding four inch stone that lives in its own handle is the most sensible object to keep in a backpack, and this one has been a field standard for years. Each face is colour coded, yellow for 325 grit coarse and orange for 750 grit fine.

Both sides carry a micro-sharpening pad and a groove sized for fish hooks and other pointed tools, which is the feature that justifies it for anglers and field users specifically.

The interrupted diamond surface is not decoration. Instead of a continuous layer of abrasive, the surface is broken into small raised islands that hold the metal filings away from the working area, which keeps cutting fast and reduces clogging.

The rubber grip and thumb guard make it safe to use in the hand, and the drainage ports in the handle mean a wet clean-out does not sit in the case.

Can it fix a dull knife?

For pocket knives, folding knives and small fixed blades, yes. The 325 side sets an edge quickly on something that has been cutting card and rope, and the 750 side gives a usable working finish.

For a full chef knife it is the wrong shape. Four inches long and two wide, you would be repositioning the blade constantly, and a long bevel does not fit comfortably on the face.

What should you watch for?

It takes 22.68 grams, which is heavier than some folding alternatives of a similar type. A few recent buyers also reported thinner diamond coating than earlier production, so inspect the surface on arrival.

Cleaning slurry out of the interrupted pattern takes a brush rather than a rinse, which is the main recurring complaint from owners. Budget for that habit and the tool stays fast.

9. DMT FWFC DuoSharp – Best Portable Diamond Sharpener That Still Cuts Fast

This is the plate to buy if you want real DMT diamond without a full benchstone setup. Same micronized monocrystalline surface as the three-stone kit, same fast cutting action, and it folds to five inches so it fits in a drawer or a tool bag.

Two stages cover most sharpening: 325 grit coarse at 45 micron to restore a neglected edge, and 600 grit fine at 25 micron to hone it. That is enough for kitchen knives, chisels and hand tools.

It sharpens dry or with water and needs no oils, so cleanup is a brush and a wipe. That single property is what makes it the practical choice for anyone who resents sharpening maintenance.

Made in the USA with the same diamond process DMT has used for over half a century, it is also the lightest item in this roundup at 75 grams.

Is a folding stone as good as a flat plate?

The abrasive is identical, so the result is identical. What the handles change is the working method: you hold the blade in one hand and move it, rather than sliding the knife across a supported surface.

That is a real trade-off for consistency on a beginner, since freehand sharpening depends on a steady hand and a steady hand takes practice. Experienced users at r/sharpening treat the folding format as the ideal travel setup.

What is wrong with the handles?

This is the most common complaint and it is not minor. The plastic handles stop short of the centre of the plate, so the middle can flex under pressure, and several owners say the newer plastic is less durable than earlier versions of the same folding design.

Use light pressure and you will not notice. Press hard on a long blade and you will, which is why the wider DMT kit is our recommendation for anyone doing heavy work at a bench.

10. S SATC 8 Inch Diamond Plate – Best Value Plate for Kitchen Knives

This is the plain diamond plate: a precisely machined steel plate electroplated with monocrystalline diamond, one side at 400 grit and one at 1000. Nothing decorative, no case, no guide. Just an abrasive surface that stays flat.

That flatness matters more than any accessory. A whetstone dishes in the middle after a year of use and stops cutting evenly, while a bonded diamond plate keeps working area after area.

At eight inches by 2.75 inches it covers the length of most kitchen knives without constant repositioning. The 400 grit side is coarse enough to set a new edge on a dull blade, and the 1000 side is fine enough to leave it presentable.

It also handles straight-edged tools beyond the kitchen, including chisels and planer blades, which makes it useful in a workshop as well as a kitchen drawer.

Is 400 grit enough for an extremely dull knife?

For dullness, yes. Forty micron diamond is roughly equivalent to a 325 grit stone in the DMT kit, and that is the grit most people use to bring back a neglected edge.

For real damage it is marginal. A deep chip or a rolled tip wants something coarser, or a different approach entirely such as a regrind. If your edge has visible gaps rather than general softness, start finer down the list.

What do you give up?

The profile at 0.3 inches is thin, and thin plates flex if you lean on them. Hold the plate flat on a damp cloth and use light strokes, which is also the advice forum regulars give for any diamond plate.

There is no base and no angle guide in the box. That is fine once you know what you are doing, and less fine if you are starting, so pair it with a folded towel for grip while you learn the angle.

11. KITANIS Five-Piece Set – Budget Diamond Sharpening Stones With Every Grit

Most people who start sharpening assume they need three or four stones. This set gives you five at 400, 600, 800, 1000 and 1200 grit, each with a patterned honeycomb surface, which is the widest grit coverage here for the least money.

The honeycomb is the useful feature. Swarf falls away from the working area instead of packing into it, so the stones keep cutting at a steady rate rather than glazing over halfway through a session.

A non-slip base is included, which more budget sets skip, and the whole package weighs 350 grams. That makes it easy to keep in a drawer and easy to move between the kitchen and a workbench.

The manufacturer suggests a 15 degree angle and light pressure, and adding water or honing oil improves the feel of the grind considerably.

Will it handle a really dull knife?

Yes for dullness. The 400 grit stone is the right entry point for an edge that has lost its bite, and you can climb to 1200 for a polished finish whenever you want one.

Nothing here is coarse enough for a badly chipped edge, and the thin plates make sustained pressure awkward. Both push you toward the DMT kit or the S SATC plate if your knives take a beating.

What are the compromises?

Each stone is 0.04 inches thick and 5.9 by 2.48 inches. That is small and light enough to flex if you press down, so support it on a flat surface and use the stroke rather than the force.

The brand is not established in the sharpening world, and flatness consistency over years is unproven. Treat this as a starter set to learn on, then graduate to a heavier plate once you know what angle you prefer.

12. Ultra Sharp Three-Stone Set – Best Large Surface Area for Long Blades

Surface area is the spec nobody quotes until they have sharpened a full chef knife on something small. Each stone here measures eight by three inches, so a whole blade lies flat on the abrasive in one pass.

The progression runs 300, 600 and 1200 grit. That 300 grit start is the most aggressive coarse grit in this roundup, which makes it a good fit for edges that need a lot of steel removed before they are sharp again.

It is one of the few sets here that is claimed to work on ceramic blades as well as steel, so owners with ceramic knives are covered. The stone holder keeps plates stable during a session, which is the piece most sets leave out.

Each stone weighs 2.2 pounds and the package comes to 8.5 pounds. That is a bench, not a drawer, so plan where it lives before you buy.

Is it right for an extremely dull knife?

Among the most capable on this list. The 300 grit stone opens a neglected edge quickly, and the wide surface means you can work the full length of the bevel without stopping halfway to reposition.

The wide plate also reduces the risk of rounding the tip, since the abrasive supports the blade rather than having it tip forward off a small face.

What are the drawbacks?

Weight, mostly. At 2.2 pounds per stone you will not be carrying these to a campsite, and moving one with wet hands is awkward.

There are no angle guides in the box, so you are working from feel or from a printed reference. Some owners also report differences in finishing between stones in the set. The lifetime warranty is a genuine plus that few competitors match.

How to Sharpen an Extremely Dull Knife With a Diamond Stone

A dull knife needs the same sequence a sharp one does, only with more strokes at the coarse end. The single biggest mistake is starting too fine and never getting the geometry back.

1. Start at 325 or 400 grit. These are the coarsest grits here and they remove steel fast enough to cut through a rounded edge without a hundred strokes. Once the knife is slicing paper, you have removed enough.
2. Set a consistent angle. Use the guide on the stone or holder if one is included. Western knives usually sit at 17 to 20 degrees, Japanese single-bevel knives closer to 15.
3. Move to 600 grit. This refines the coarse scratches from the first stone and leaves an edge that is sharp rather than merely sharp-ish.
4. Finish at 1200 grit. The extra-fine stage polishes the edge, which improves how it cuts and how long it holds the sharp condition.
5. Strop if you own a rod or leather. A few passes on a honing rod align the edge and remove any remaining burr. The SHARPAL 181N leather sheath works for this.
6. Clean the plates. Brush metal off with a stiff brush while the slurry is wet. A loaded diamond plate stops cutting and starts polishing.

Light, even pressure beats hard pressure. Diamond particles do not respond well to maximum force, and leaning on a plate wears the bonded layer faster without sharpening any quicker.

Before the next session, read our guide to dart sharpeners. The grit selection logic is identical, since both jobs are about removing material evenly rather than forcing through a hard spot.

Diamond vs Water Stone vs Ceramic: Which Should You Buy?

Diamond sharpeners are better than stone for most home kitchens, and worse than stone for a very particular edge. That is the honest answer rather than a slogan.

Diamond removes metal quickly, stays flat indefinitely, works dry and needs no soaking or flattening. It also leaves a slightly rougher edge than a fine water stone, because the abrasive is graded by particle size rather than by a natural bond.

A water stone produces a finer, more polished edge and can be reshaped into a narrower bevel. In exchange you soak it, let it dry, flatten it on a second stone and keep track of its condition. Users who hate maintenance rarely go back.

Ceramic is the opposite of diamond in the hardness sense. It is softer than steel, so it cannot cut hardened high-carbon steel quickly and it needs more pressure and more strokes. What it does produce is a very clean, toothy edge, and it stays flat because it does not deform.

For dull knives specifically, diamond wins because the job is metal removal and the hardest abrasive available does it fastest. If your goal is a razor-polished edge on a Japanese knife, a fine water stone remains the finishing step that diamond cannot replace.

Troubleshooting: Why Your Knife Is Still Dull After Sharpening

If the knife still slides through a sheet of paper after a session, one of these is usually the cause. All five came up repeatedly across our own testing and in forum threads.

The burr is still there. A wire edge forms on the opposite side of the bevel during coarse work. It feels sharp when you run a finger along it, then folds over and the knife feels dull again within a minute. Deburr on a finer grit or a honing rod before you judge the result.

You started too fine. A 1200 grit stone on a knife with a rounded edge will polish a dull edge and leave it dull. Go back to 325 or 400 grit, remove real material, then climb back up the progression.

The angle changed. Holding 30 degrees on a knife that arrived at 20 leaves a wedge that cuts badly and chips easily. Fewer, more deliberate strokes at a consistent angle fixes more problems than extra pressure.

You only honed. A honing rod realigns an edge, it does not create one. A knife that has lost several millimetres of steel needs sharpening, and no amount of rod work will bring it back.

The plate is loaded with metal. Diamond stops cutting when swarf fills the gaps between particles. A hard brush and a rinse while the slurry is wet restore most of the bite. If it stays slow, the bonded layer may be worn through.

You rounded the tip. Concentrating strokes on the point of the blade creates a rounded tip that shreds instead of cutting. Lift the pressure near the tip and let the edge run the full length of the stone.

Buying Guide: What to Look For in a Diamond Sharpener

Grit range decides whether it suits dull knives. For a genuinely dull blade you need a coarse side at 325 or 400 grit. A tool with only fine grit will not fix a rounded edge no matter how good its diamond is. Two sides covering coarse and fine handles most sharpening, and a third stone at 600 or 1200 gives you a proper progression.

Plate size determines how much work a session takes. An eight by three inch plate covers a whole chef knife in one pass and reduces the chance of rounding the tip. Smaller stones are more portable but need repositioning constantly on long blades. Weight matters too: a plate that slides while you work is a plate you will avoid using.

Flatness is what separates a good plate from a bad one. Water stones dish in the middle; bonded diamond plates hold their surface. A plate that stays flat produces an even bevel, and an even bevel is what keeps a knife sharp between sessions.

Check whether it works dry. Every diamond sharpener on this list works dry, and that is the single biggest convenience difference from a water stone. Oil and water are optional for lubrication and slurry cleanup, not required for the abrasive to function.

An angle guide is worth having while you learn. Guides at 20 degrees suit most Western kitchen knives, 15 degrees suits Japanese single-bevel knives, and 17 degrees is the common compromise. Once the angle feels natural you can remove the guide or work freehand.

Understand the format before buying. A benchstone gives full control and needs technique. A guided system holds the angle for you and is best for beginners. A pull-through or slot sharpener is fastest and works on serrated blades but cannot repair chips. An electric unit removes the most steel with the least effort. A rod or file is for honing, travelling and small tools.

Honing and sharpening are different jobs. Sharpening removes steel and creates the edge. Honing realigns an existing edge with almost no metal loss. Forum veterans are emphatic about this: if a knife merely feels blunt, hone it, and reserve sharpening for an edge that has genuinely rounded over.

Check the warranty and whether abrasives are replaceable. The bonded diamond layer is the consumable part. A three year or longer warranty signals a manufacturer that stands behind the bond, and replaceable abrasives extend the life of the tool considerably.

Look for a stropping option. A fine stone or ceramic plate is the usual answer, and a honing rod or leather sheath adds the final alignment step. That last stage is what makes a sharp knife feel sharp rather than merely pointed.

Frequently Asked Questions

Conclusion

Twelve tools later, the answer for most people is the KitchenIQ 50825, because it removes dullness in under a minute and needs no technique. The moment your knives have chips rather than just lost their bite, move up to a coarse plate: the SHARPAL 162N for a wide flat surface with an angle guide, or the DMT W6EFC kit for a proper three-stage progression.

Use light pressure, climb the grits instead of jumping to the fine end, and clean the slurry off while it is wet. Those three habits will keep the best diamond sharpeners for dull knives working for years, and every one of the twelve above stays flat because diamond plates never need flattening.