Using An Acrylic Plate for Sharpening or Flattening

[Don Rogers]

I have acquired some plastic cutoffs (acrylic, I think) and am wondering if one would be useful in sharpening chisels, plane blades, etc or in flattening stones.


They are about 1” thick, 7” wide and the one I am considering is about 12” long. Although I have not yet checked them for flatness, they appear to be dead flat and still have the protective paper on one side. The other side has no protective paper but looks dead flat also with no scratches.


My thoughts are to sprinkle some carbide grit or diamond paste and some oil on one side and give one a try. The grit should imbed itself into the plastic surface. It would seem that it may work. I have heard of pasting Mylar film on plate glass so that the grit imbeds into it. I understand this works quite well.


However, like any surface used in a similar manner, the plastic would eventually wear out-of-flat and then require re-flattening. Resurfacing it would be extremely hard on almost any tool because of the imbedded grit or diamond paste. Unless this problem is solved, using the other side would be the only way to get more use out of it. Eventually. both sides would have imbedded grit or diamond and the plastic block may have to thrown out.


What are your thoughts about using this plastic for sharpening?

[Mike Brady]

I disagree. Acrylic will degrade too quickly with loose grit applied to it. Sandpaper adhered on that thick a surface might be ok. Loose grit is best used on cast iron where it can embed in the surface and break down during the lapping process. It would be very difficult to flatten acrylic once it does get abraded. Loose grit on a surface as hard as granite might work and small pieces of flat granite can be found at countertop fabricating shops. You seem to already have a pretty good idea of the limitations of an acrylic matrix for lapping.

[Reinis Kanders]

I just did that with a 1/4" plexiglass and 40 micron diamond paste on a surface plate to lap a back of the iron. It was pretty fast, but not that fast, nice polish though. I have a big piece of plexi so I will just cut another plate out of it once it goes out of flat. Woodcentral archives have a lot of interesting info on lapping and sharpening with diamond paste or lose diamonds.

[Andrae Covington]

About a year ago I bought some 1/4" acrylic pieces and tried sharpening with silicon carbide grit and mineral oil. Maybe it was the relatively coarse grit (80 I think) I experimented with, but it did not seem to embed easily, so the blade mostly pushed it around until there was not much left to do any meaningful cutting. The mylar film idea would probably help.

It's messy, you might want to work in a shallow tray or something. Also, loose-grit sharpening is better suited to freehand than guided IMO, as the loose grit can chew up the wheel (particularly if it's brass) on a honing guide. Wonder how I found that out? :(

Later I tried some 1 micron diamond paste on the acrylic. That worked better.

I'm awaiting a package from LV with some PSA diamond film which I plan to affix to the acrylic plates and see how that goes.

[Tom M King]

Try to catch the surface plates on sale at Woodcraft. A couple of times a year they sell the 9x12 for 25 bucks. The diamond film works great, but it is fragile. It cuts really fast, but you have to be careful to only pull back on it, or the quickly sharpened edge will slice right down through the film. I bought one of the granite surface plate specifically to cut up on the brick and tile saw for the diamond lapping film pieces. I like the flat, weight, and height, but having four pieces of the film on one was not the ideal handling setup for me.

I use sandpaper on granite surface plates for flattening, and never liked loose grit.

[Brian Holcombe]

I bought one of the granite plates for woodcraft (9 x 12) and it's great. I think I paid $50~ including shipping (I tend to have poor pricing recollection when it comes to woodworking tools :D).

I use mine with sandpaper, cheap stuff that I paid about $.35/sheet. For the low grits cheap stuff works fine for me since I'm consuming one for every blade. I have as low as 80 grit but often will start higher (220) if the blade is not that far out.

[David Ragan]

I think the acrylic is too soft. It may seem cool @ first cause the grit may imbed in the acrylic, but it will likely glaze over, or, unless there is continuous water cooling, heat up a little and deform.

Why not experiment, find a way to monitor for absolute flatness, and let us know how it works.

[Don Rogers]

Thanks to every one for his input. I personally thought this approach - using silicone carbide grit on a flat plastic surface - would not be practical but wanted other opinions first. Using diamond paste for polishing would work and using sandpaper would be less destructive to the plastic surface - but a harder material such as glass or granite would be even better.


I have a plate glass piece and a 5“x 36”granite countertop piece to try also but really wanted to find a good use for the 1” thick offcuts.


Mike, we really agree on everything in your post but perhaps i was not clear in mine.


Renis, your use of 1/4” Plexiglass with 40 micron diamond paste lets me know that it works for polishing. However, my conservative nature will not let me discard a 1” thick piece when it is no longer flat. Maybe re-surfacing it on a sandpaper covered glass or diamong plate will permit it’s use again


Andrae, that’s good information and I won’t need to try silicon carbide grit and mineral oil on Plexiglass . Also, using a mechanical chisel or plane blade sharpening device would be very hard on the roller. Hand held would be better. Let me know how you make out with the PSA diamond film on the acrylic plates.


Tom, I have a plate glass piece and a 5“x 36”granite countertop piece to try. (also a Starrett Crystal Pink 12 x 18”Surface plate which I don’t want to get any kind of grit on). Your experience with the diamond film is very interesting. I may give that a try.


Brian, Good information. I seem to prefer the sandpaper on glass or granite also. Basically, with all the sharpening and flattening methods available, I can easily become confused. Someday I hope to settle on one system and get rid of everything else. For now I will no longer consider loose grit and oil on the 1” thick acrylic - but I still hope to find a good use for it.

[Dave Anderson NH]

A couple of cautions other than those the others have mentioned. 1" thick acrylic is likely to be cell cast material and as such it has a large variation in its thickness through out the sheet. Find an accurate straight edge and check out the flatness side to side and diagonally by using either feeler gages or holding it up to the light. Thinner acrylics are usually more accurate in thickness since they are either melt calendared or extruded. If it is accurate, it should work fine, but its life will be relatively short before it gets dished, gouged, and the surface becomes uneven.

[Don Rogers]

David, I agree with everything in your post and will try to find the time for experimentation.

[Don Rogers]

Dave, Nice hearing from you again.

I measured the acrylic thickness around the edges of the 7" x 12" sheet using my 1" micrometer and found a max of 0.006 difference. The thickness measures around 0.996. With my eye problems, I was unable to estimate its flatness but did see some small amount of light showing under the straightedge. Not very accurate but it's obviously not dead flat. Iv'e given up hopes of using it as a lapping or honing plate but mostly because the embedded grit would ruin it for other uses.

Some time ago, I surface ground a plate of 3/8" x 3" x 6"steel (soft but unknown material - probably CRS) for a honing plate but have yet to try it out. Although small, I believe it better to try it rather than the acrylic.