phosphoric acid/paint

[Mark Whiddon]

Hey Guys
I have searched the archives and have not found the answer to this question yet. I posted a question concerning rusting under a truck about a week ago and got LOTS of good info. the rust on the panals is only just started and is underneith, not visible . My question is can I paint on phosphoric acid DIRECTLY onto the already painted surface and then paint over it with something like paint on bed liner, or undercoating, or would it be better to phospho/prime/paint? Or would the phospho eat the paint that is already there and totally ruin what is already a decent paint job, just have some very small rust spots starting to form on the body panels underneith? My plan was to phospho it then to put either undercoating or roll on some paintable bed liner. Am I just going to waste my time? Or would this be benificial. Like I said, it does not have big holes, just some very small rust spots forming. Thanks guys
Mark

[Mike W]

You could try a small area. I am guessing the acid won't bother the paint. Since it is underneath if you make a mistake, just sand it and put some paint on.

[rjgogo]

You are obviously not from the Midwest.
New cars and trucks have rust here from the factory.

Bed liner over rust, even a little will just trap water and salt and make it worse.

Do what you can with a little paint and a wire brush and hope it works.
I hear good things about POR but no experience.

[Franz]

There's some fairly intensive discussion of the phosphoric process in the thread on electrolytic derusting on the projects board.
Phosphoric won't harm good paint, but it will work under paint around rust, and lift that.
Keep your acid concentration below 10%, and keep the rust wet long enough for the acid to work all the way thru the rust layer, or you'll just be wasting time and acid.
Usually when you have rust showing on lower body panels, it has worked it's way thru from the inside of the metal, and just treating the visable rust won't help much.