Phosphoric Acid Rust Treatment

[ramjam2005]

Has anyone used phosphoric acid for rust removal on body panels ? In particular, after soaking did you neutralize or wash off the acid before re-applying a primer to the bare metal?

I purchased several new body panels / repair panels many years back but due to the stalled restoration and lack of suitable storage these have developed various levels of surface rust patches or rust pitting breaking thru the Red oxide primer or E-coating.
I want to treat and address the spots and pitting BEFORE fitting any of these panels, having decide to eventually use an Epoxy primer to a bare metal finish. (already researched pros and cons of primer choices etc).

I've previously used a Phosphoric acid bath on small suspension components with good results before applying a rust protective paint and/or POR-15, but in warmer dry weather. So today I tried similar on one of the smaller affected body panels, the top front apron, by stripping any remaining red oxide / e-coat using a poly-carbide stripping disc then submerging it in a shallow bath I made filled with the phosphoric, agitating any pitting with a small wire brush before leaving to soak for an hour or so resulting in a good grey coating.

I've been doing plenty of research and reading across the web on panel preparation using this method and most seem to say after the soaking to neutralise by rinsing off with bi-carb of soda dissolved in water or plain water then dry. I followed this process but as soon as I dried off with a lint free cloth it immediately started to show mild streaks / tint of flash rust on the clean grey surface and in the treated pitting. The cold, damp evening in the garage probably didn't help either.

So should I simply wipe dry the panel after its soaked and NOT try to neutralise with water as the initial grey coating looked good ? I was thinking, maybe just cloth dry any remnants of acid, maybe near a warm heat source, then use panel prep degreaser (minus the heat source of course ) before applying the primer coat.

I eventually want to use an Epoxy primer on bare metal so I have concerns of using other rust treatments that go on and stay on in case it compromises the benefits of Epoxy adhesion. Any minor pitting will be filled by the epoxy and/or future coats of high build during the final body prep stages.

Any advice welcome.

Ray

[jbuckl]

Have used 85% phosphoric acid for de rusting.

Have used acetone to clean up afterwards.

Recommend testing any such efforts on a small area / seam before committing to fully painting.

[flying farmer]

I have also used phosphoric acid to de rust parts, usually followed by a good wash with a hose, but you do need to dry the bare metal parts very quickly as rust will rapidly reappear. I usually blow most of the water off with an air line then dry with a fan heater, preferably in a warm room

[ramjam2005]

I have also used phosphoric acid to de rust parts, usually followed by a good wash with a hose, but you do need to dry the bare metal parts very quickly as rust will rapidly reappear. I usually blow most of the water off with an air line then dry with a fan heater, preferably in a warm room

Thanks for the replies guys.
This was my first attempt on a larger flat panel and having hosed it off, walked back into the garage where I had a heater on wiping it dry over the heater and saw the flash rust appear as I dried it.
I tried cleaning off the flash with panel prep cleaner which removed most of it but could see it had re-appeared in the pitted areas after the panel prep evaporated.
There seems to be two camps on various threads/forums I've read, those that say rinse it off, those that say don't and safe to primer over phosphoric once dried on. It's the pitting that's my main concern as I don't want the rust coming back to bite my bum a few years later having invested time and money on achieving an excellent paint finish.

[torque2me]

Thanks for the replies guys.
This was my first attempt on a larger flat panel and having hosed it off, walked back into the garage where I had a heater on wiping it dry over the heater and saw the flash rust appear as I dried it.
I tried cleaning off the flash with panel prep cleaner which removed most of it but could see it had re-appeared in the pitted areas after the panel prep evaporated.
There seems to be two camps on various threads/forums I've read, those that say rinse it off, those that say don't and safe to primer over phosphoric once dried on. It's the pitting that's my main concern as I don't want the rust coming back to bite my bum a few years later having invested time and money on achieving an excellent paint finish.

I guess a little experiment is called for. My use of acid has always mentioned the rinse or damp cloth/sponge afterwards but to provide more data you could do a panel with the clean water wash and one with leaving the "inert" (is it ever?) phosphoric acid coating. Keep us posted on what you decide/further actions.
Kev

[ramjam2005]

I'm hoping to speak with a couple of guys at the auto body repair company I drove for. They worked solely in the restoration unit and turned out amazing work / cars. Hopefully they'll share how they approach similar rust damage.