There have been a few posts about using phosphoric acid for derusting. I've
been messing with the stuff for a few years now and have the following
tips.
It seems safe enough, but I still recommend using protective gloves and a
face shield when handling parts.
I still think that the solution when used should be disposed of as
household hazardous waste. The residue from derusting car parts (paint and
other gunk) has to be bad for the environment. (The tips about pouring
phosphoric acid in the garden reminded me about an article I saw about
Fairfax County, Virginia. The number one source of water pollution in
Fairfax County was point source pollution. That is cars dripping oil,
lawnmowers, antifreeze leaking, red necks changing oil and draining it in
the woods, etc. Fairfax is lucky in that it has no real heavy industry.
There's no need to add grease, paint and other stuff to the problem. Take
used acid to the appropriate waste facility.)
Be careful what you put in a phosphoric acid bath. Chrome pieces after a
few hours obviously will no longer be chromed. Non-ferrous metals don't
like it either. Aluminum makes some neat bubbles. I wouldn't put springs in
it. Tom Brownwell, in his book "Restoring Old Cars," says the phosphoric
acid causes hydrogen embrittlement which weakens springs. I found out the
hard way. I would also be careful about any part that has a surface with a
surface finish requirement (like brake drums, calipers, etc.).
Watch the parts and don't leave them in the solution too long. As soon as
the rust is gone, it seems like the metal starts going too.
After the stuff sits for awhile, some residue precipitates out of it. To
keep the solution more active, decant from one container to another trying
to avoid the getting precipitate in the fresh container. Always use a
plastic container for the solution.
Phosphoric acid can also remove paint from parts. The primer usually
requires some wire brushing. It's a little gory but effective. But now that
I've built a sand blast cabinet, I prefer to blast the parts and then
quickly dip in a solution of phosphoric acid and alcohol. The blasting
gives a nicely roughed surface and the acid etches and partially stabilizes
the part before finishing. Seems quicker to de-paint, de-rust parts and
prepare parts for finishing this way.
Have fun and be careful.
Regards,
Rob Kempinski Sunbeam Tiger Mk 1a
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