Quick Tip #7: Sharpen a Rasp

Rasps get dull, just like any other tool. Most people just pitch them when they get dull and buy a new one. But before you go tossing those olds dull rasps, or grinding them up into whatever, give sharpening them a try. No, I’m not talking about honing all the teeth with a microscopic stone. Even I’m not that nuts. To sharpen a rasp, we turn to chemistry. Blacksmiths have been using this trick for sharpening their files for decades. All it takes is a few inexpensive items from the hardware store, and some salad dressing.



21 thoughts on “Quick Tip #7: Sharpen a Rasp

  1. It may also be useful to perform a second rinse, as the reaction which neutralizes the acid actually creates salt. Rinsing this salt away with hot clean water may help to further protect the tool. Using hot water will warm the tool, and help it dry itself if you aren’t using a hair dryer or heat gun.

    • James,
      A good tip indeed. While my background is in chemistry, I tend not to think about it any more than I have to :). I haven’t had any problems with residual salt rusting the tools, but it is certainly a potential.

    • Rob,
      Thanks for the link! Your method for cleaning rasps sounds like a winner. I’m also glad to hear the Michel supported it. That’s one to file away for the future!

  2. Great video Bob. One additional tip, that is probably more useful with files but could be applied if needed to rasps: If the file or rasp has a bit of oil on it, you want to remove that before putting it into the acid. I use a spray bottle with a mixture of ammonia and water. Ammonia is a great degreaser, so I spray the file with it and use to remove any oil or residue that would keep the acid from being able to get to the metal. Keep up the great work!

  3. Your sharpening technique seams very practical. You also mention that there are regrinding services. Could you share the names with us? Will they work on random toothed files?

    Thanks for all your help. You do a great job!!

    • Boggs Tool sharpens rasps and files. They don’t grind though, they chemically sharpen, similar to what I did. Rasps and files are never ground. I don’t even think it’s possible to do so.

  4. Bob, do you think this process would also work on the Grammercy Rasps? they are stainless steel.

      • I am trying to decide between a grammercy and aurori rasp, each has some good points, this might sway me toward the aurori

          • thanks, spent the past week in Colonial Williamsburg. I spent as much time as my wife could stand in the Cabinet and Jointers Shops. I breathe the air like a hyperbaric chamber, and try to have it infuse my very soul. I will prob pick up the Aurori rasp after Christmas.

    • Thanks Harry! And I certainly will at some point when they’re ready for sharpening again. I’m sure you guys do it much better than my home remedy. These days, though, I find myself using rasps less and less in my work, preferring edge tools like shaves, chisels and scrapers instead.

  5. For stainless files you may need a slightly stronger acid. This is from my experience in the steel industry. When I worked for a carbon steel manufacturer, they were using HCl (muriatic acid) – however HCl only removes oxide, it doesn’t etch away very much steel. strip could sit in an acid bath for a long time and not have any appreciable damage (gauge removal). They used to use sulfuric acid – that etches away steel, but you need good ventilation unless you want nosebleeds and your teeth to turn green. Most mills stopped using sulfuric for safety reasons, and the problems that occurred with too much metal etching away if the line shut down unexpectedly.
    Stainless steel is usually pickled in a HF acid and nitric acid mixture. Some places use Peroxide now due to environmental reasons over the nitric acic. HCl doesn’t do much if anything to the oxide layer on stainless. HF is very dangerous to handle and you would probably have a difficult time finding any. Nitric needs to be diluted a LOT for carbon steel. It will definitely etch away carbon steel and probably stainless. Not sure if you can find any easily.

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