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How to sharpen a file.

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There was one step that did not seem to be presented:

Any sharpening would occur due to the acid dissolving an even layer 

of iron on the file.  So the acid must have access to the surface of the metal.

Any wax, grease, or oil on the surface could occlude the water from the metal.

A pre-cleaning with detergent, water rinse and mineral spirit tx would give a fresh

iron surface.


The mineral acids discussed can be potentially dangerous and a problem to discard.

I am wondering ( as a denken experiment ) if electrolysis would not be a safer method?

The file could be wired as the donor and a copper rod to accept.  I can't recall ever 

seeing iron being used to plate another metal, but it should behave as any other metal.

The slightly salty water medium should be no problem to discard.  I have not looked it up,

but I think that since Fe has a positive charge, the copper rod should get the negative charge.

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Good point Jaeger about thoroughly cleaning the file.  

Another source of H2SO4 (sulfiuric acid) is the automobile parts store, where I got a supply of battery acid.   

Whatever one does, be careful with this chemical.                               Duff  

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I introduced myself to one property of sulfuric acid inadvertently.  It is intensely hygroscopic.

That is - it readily combines with water.  It takes a lot of energy to remove the water, so

when it does combine with water it gives back that energy as heat.  On your skin, it feels like

a jet of live steam has hit it.  Adding water to acid, the water instantly turns to steam and blows

out of the liquid - taking some of the liquid with it.   I have never added water to acid, but I 

did discover that reagent sulfuric acid is thick and does not pour like water.  It tends to come back

down the surface of the container it is poured from.  Pouring it from a beaker with your thumb on

the bottom of the beaker is really not a good way to do it.

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In German we have a saying: "Erst das Wasser, dann die Säure - sonst geschieht das Ungeheure" - in free translation "First the water, then the acid - otherwise you are in deep ****". The first drops of water vapourise immediately and the steam carries with drops of acid ... I am sort of chemist, a geochemist to be precise, and I know what I am talking about. Guess why - well not because I made the mistake, but I have seen others doing it and nearly hitting me with a spout of hot sulfuric acid ... :o


I think the electrolytic method would be much safer and more controllable. The other electrode needs to be more inert than the iron (conversely, one uses less inert metals, such as zinc, to protect iron/steel ship parts). I would doubt, however, it is worth the effort. As was noted above, the surface would rather be eaten away quite uniformely, while in sharpening you would need to eat away the bent-over and flattened teeth. Before the process you would also need to clean the file from any dirt, grease, stuck-on filings, etc. in order to present a metallic clean surface.


It is better to maintain the files carefully and to buy a new one every 20 odd years or so ...

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Think buying new every now and then is the way to go, although I had a friend, now gone, give his files the acid treatment, he liked the result. Buying new and throwing the old in a box has been the norm around here for a very long time and there must be over 60 pounds of old files, rasps and hoof files collected. Think I will keep them, they don't take up much room and never know when I will need to make a good knife.


Edited by jud
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Buying new files is much safer, and taking good care of them is excellent advice.  

Files should be stored in such a way that they do not bang into each other.  

I store mine in a wood drawer with thin wood dividers, and some have card board sleeves slipped over the teeth.

My micro files are also stored with corrugated card board separaters and the really small ones are pushed into the the end of the card board.


Old files can be made into excellent knives and turning chisels.  Be careful not to draw the temper while grinding.       Duff

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  • 5 months later...

I tried the sulfuric acid trick forty years ago. It improved slightly but not like a new file.

After a few more etches, and a closer examination, I found that etching rounded file teeth still leaves rounded file teeth, yet smaller.

Some files are very hard on the surface, yet softer underneath.


I hardly have to buy a new file. I always try to use the right file for the job. If I don't, a file can get blunt in one single swipe.


I prefer to buy a new file from a reputable industrial supplier (not hardware store).

Then I take the old file to the grinding stone and take the teeth off at one end. Leaving square edges. This turns into a powerful scrapper.

I have taken off the teeth along the thin edge of a file to use for filing into corners. This allows me to only file one edge at a time, while the other (blunt) edge simply slides along the surface.


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35 minutes ago, Tecko said:

... I have taken off the teeth along the thin edge of a file to use for filing into corners. This allows me to only file one edge at a time, while the other (blunt) edge simply slides along the surface.

This is called a safe-edge in the trade and many files can be bought ex-factory like this.

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  • 2 weeks later...

I guess it depends on in which direction you use the tool and on the shape of the file you use. I would have to read up again on file-making, but believe that after being hewn, they would be tempered to destress them before hardening. So there is not that much reason for them to break along the teeth lines really. Perhaps so, if you use really cheap ones. However, files are really hard and therefore brittle. If you turn thme into turning tools they would need to have some hardness drawn from them so that they are less likely to break.

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An interesting discussion to which I'm a late arrival! 


Wefalck's comment about the saying in German reminded me of my college chemistry teacher's comment, which works best if you say it with a Brooklyn accent "Do as you oughta, add acid wata".


Always wondered what to do about files getting old. I like the "buy new files" idea.



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Ahoy Mates


I have used files that were "sharpened " with acid. They are fine for soft things like balsa,but the acid thins out the "teeth" and anything harder just dulls them very afst.


Lesson  learned-Buy good ones to begin with and treat them like I was told by a Master Tool Maker-my Dad when I was old enough to start using his files at home.

Keep them clean-brush the teeth out with a BRASS BRUSH,and DO NOT PUT THEM AWAY IN CONTACT WITH METAL OBJECTS_LIKE OTHER FILES. He had drawers with wooden drawer with wood dividers that only one would go into each slot. O)r have a leather or canvas roll up with slots for each file,so that they are protected.


Treat them as what would be the way a chef stores  his expensive knives. In a roll up. That way when you buy an expensive file that could last you years if you use it as it should,it WILL.


Cheap chinese or other low grade files are just what they are-cheap. But even those should be handles this way. And they too will last longer,but not like high grade ones.


I now use diamond files for a lot of my model work. The thing I like about them is that they can be used in any direction,where as files cut in one direction,except ofr some types.


High grade files are not easy to find these days except for quality tool stores. When you find the style and maker you like,stock up on them for future use. Because you never know when they might not be for sale any more due to closure of the maker or the place where you buy them quits carrying them due to lack of sales with competion of the crap cheap ones.

There are many styles to chose from. Learn which ones will be best for you and write down what works best for you,and where to buy them.


I have files that are over 60 years old,and still use them. Yes they do get dull,but making models is not like filing on D2 tool steel each day. 


One trcik you can use is that if you are filing aluminum or steel,take some stick of chalk,yes what you use on a black board to write with,and load up the file teeth with it before you use the file. This will prevent the teeth from loading up solid with the material you are filing,and make it easy to clean after use. It will also prevent chips from loading up and marking the metal you are filing.


And NEVER OIL A FILE! or put WD40 on one. This will only make the file slide over what you want to cut. If you are worried about the file rusting,keep it in a temp controlled area,where there is not a lost of humidity where temp changes will cause moisture to condense on the metal surfaces. A unheated garage is not a good place to keep files.

And do not wipe your hand over metal that you are filing on,your hand oils will be just like putting oil on it. But if you do,clean it off with a degreaser before filing again.


After learning this from my Dad over 50 years ago now,it has been one thing of many that he was totally right about.

The money you would spend on acid and neutralizer and gloves and containers to acid sharpen your cheap files will be better spent on good files to begin with. And you will not be in any danger from use of acid,and then you have to dispose of it safely.


Just but good files to begin with.



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Very interesting discussion. I have two related questions:

1. Is there a way to clean out the very small files we use in model making? A normal file card is way too coarse.

2. Can someone name a source of really good small files for modelling? The Otto Frei dealer mentioned by vossiewulf is a good one; i get many supplies from them as they are handy to me here in Oakland. But their files are really expensive. Maybe that's the only way to get good files--pay big bucks for them.

Thanks for any ideas,



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Hello Les, back here on this issue. When a file is dull it is dull. Why waste time with acid etching or what ever process? Go to a hardware store and purchase new. Not expensive. Unless you paid $400.00 dollars on that file, as a carpenter once again here's what I do. Recycle that thing. The  health issues  associated with acids and anything else is not worth it. As I did purchase a very good set of riffler files and other wood files. They will carry you a long way. All files wear out PERIOD. If when you have to clean, soak in lacquer thinner and scrub with a brass brush. This doesn't compromise the sharpness of the file. Hope this helps.

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  • 1 year later...

I was a horseshoer for 40 years. I used files and rasps to work on horses feet. I would use it first to trim and level the hoof. When the file or rasp got tired of that job I would use it finish the horse's foot. Doing this job was half going over the hoof wall and half going over the nails and edge of the shoe. Kind of half hoof and half metal. When the rasp or file got tired of that  job I used it on metal when making horseshoes. When it got tired of that job I used the old rasp or file to make things out of like knives. I got a lot of use out of one file that way. For modeling I would use the file first for wood. Then went it got tired of that I would use it on metal. After that you are pretty much done with it. Remember a new sharp file will do the job right. Once it is dull all you are doing if buffing the wood.  Hope this helps. 

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