Using phosphoric acid (1 Viewer)

[Strait Cruisin]

I've been doing some experimenting with it.

The driver's side floor pan of my -40 has a lot of surface rust.

I first stripped the remaining paint with a chemical stripper. I then sanded with my D/A and 60 grit.

With each application of phosphoric acid (I've done 3 24 hour soaks so far), it leaves a dark scrap-able residue (ferric phosphate, FePO4). It appears I could do a couple more of applications before I'd say all the darker discolorations in the metal are gone.

I've read where you can basically paint directly over the phosphated surface but it seems to me the surface is rather rough from the etching process and its going to need some polishing up.

Seems to me too that polishing it would explose bare metal again to flash rust.

Any opinions?

 

[Icantdive55]

I've used it quite a bit and saw the same thing doing my running boards. after several (5+) applications I used a dremmel tool to get the little black dots out of the pits. As far as the surface prep goes I follow the directions on the Acid and then painted over the metal with master series using a brush. I will follow up with a light sanding and then spray on the final coat of master series. So far I am impressed with the acid for eliminating rust. I've used it on all my parts, fenders, hood, tailgate, windshield frame, running boards and all the things that mount to them. The only body part I haven't conquered yet is the tub itself. I painted over the rough surface without prior sanding.

 

[spotcruiser]

I use it a lot. You want to leave the phosphated surface, but you do need to wash it down well after application. Excess phosphoric acid has a tendency to weep through a final paint coat and leave a white residue, so I just clean the treated surface well with lacquer thinner and then apply primer/paint.

 

[jimothy]

Also remember if you're using some epoxy primers (e.g. SPI) you will need to thoroughly neutralize the acid before painting.

 

[Strait Cruisin]

Also remember if you're using some epoxy primers (e.g. SPI) you will need to thoroughly neutralize the acid before painting.

I am using a urethane primer.

What do you use to neutralize it?

 

[jimothy]

I am using a urethane primer.

What do you use to neutralize it?

I believe you need to wash it off with soap and water while it's wet. If it dries you need to reapply the acid to activate what may remain and then wash off. YMMV, I am new to this as well.

 

[Strait Cruisin]

I believe you need to wash it off with soap and water while it's wet. If it dries you need to reapply the acid to activate what may remain and then wash off. YMMV, I am new to this as well.

Yes, I've noticed that reapplication of the acid will dissolve the black residue. I haven't tried washing it with water while still wet with acid, perhaps that will leave a smother surface.

I'll put on my 'mad scientist' goggles and do some more experimenting.