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MG MGA - Phosphoric acid

Has anyone used phosphoric acid to get rid of rust on bodywork? My MGA coupe has now been grit blasted back to bare metal and I'm using lead loading (body solder)to cover my weld repairs. I've found that the flux is causing the surrounding metal to rust, and someone suggested phosphoric acid to clean the leaded areas. This has worked, but the acid leaves a white coating on the steel. Will this cause further problems when I apply primer?
Regards
David
D Jacobs

David, the acid has created a film of iron phosphate which is a great base for paint.
you should wash off the white stuff with hot water and dry off. It is probably conc. acid which is very gelatinous, almost solid. Depending on the formulation you used it may just be filler material to make the acid paste-like. The phosphate coating will still be there unless you abrade it.
Art Pearse

A lot of commercial rust converter products use phosphoric acid as their main ingredient. I also got some white residue, but mostly it turned the rust and surrounding metal dark blue or black when it was through working. The white residue would wipe or wash off, but the dark stuff was there to stay.
B Young

I coated my sandblasted coupe with liquid P/A and it lasted for a year without any rust. When the painter got it I do not know what he did but he did a great paint job. Mine was liquid P/A that I mixed into a spray bottle. I coated the whole car. It leaves a greyish colour finish on the car.

Gordon Harrison

I use phosphoric acid a lot and I think it's great. The white deposit is anhydrous phosphoric acid and I like seeing it on the steel. After using the acid I deliberately don't wash it off thoroughly from the metal but just use a damp rag and give it a quick wipe. On drying the white film of dried acid is left. This protects the metal from rusting because any moisture subsequently condensing on the metal will redissolve the white film and the resulting acid solution will prevent any iron oxides from appearing.

To get rid of the white film before painting I just reapply fresh acid and wash very well to remove it all but the steel has to be dried quickly - I use a hot air blower - otherwise a rust film will appear before it's completely dry. I think in the commercial PA rust removers they add alcohol to speed up the drying.

The grey protective film which is left on the steel is one of the iron phosphates and if the conditions are right you can get a black film which is a different iron phosphate.

The acid in the flux is almost certainly hydrochloric which is deadly stuff for causing rust.

If you've heard of people using Coca Cola to remove rust it's because Coke contains phosphoric acid!!

Mike
m.j. moore

Thanks Guys
That info is very useful. I have noticed that phosphoric acid is a main ingredient of off-the-shelf rust removers, but PA is much cheaper in its basic form.
Regards
David
D Jacobs

I'm a big fan of phosphoric acid and use it a lot also. I bought a quart (about $6.00 US) about 2 years ago from Advance Auto Parts (made by the Purple Power cleaner folks--I think it's called Purple Power De-ruster) and still have half a bottle left after using it on nearly everything I have that's bare metal.

After applying it to 2 front wings on one car, it has prevented rust for 2 years her in the South where temperatures and humidity can reach extremes. I even sanded one wing with 200 grit paper after application to see if it would be affected and after 1 year it still is rust free. Apparently, it really gets down into the pores of the metal.

I don't particularly care for the excessive white residue effect and have found that by applying with a cheap paint brush, letting it sit for about 10-15 minutes and then wiping off the excess with paper towels or rags, it leaves a really nice silver/gray, even finish that won't rust.

Probably the best $6.00 I ever spent.
JM Morris

This thread was discussed between 09/05/2011 and 11/05/2011

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