Surface rust after using phosphoric acid

[Thrasher22]

I soaked my c-pillar in phosphoric acid earlier this week (can't remember the brand name off the top of my head), let it sit for 24 hours, then washed/wiped it down and gave it a scrub with a metal brush to get any extra loose stuff off and get it ready for a second coat.

After leaving it to dry inside my house for a couple days, I noticed 2/3rds of the bare metal I hit with the acid is covered in surface rust. Am I doing something wrong? I don't want to paint over rust and sanding inside the pillar isn't really an option...

Its a very dry climate here, so metal doesn't otherwise rust in a couple days.

Or am I just being over paranoid?

[Mike Fisher]

I would never use water or a wire brush on acid etched bare metal. You scrubbed all the acid off of it. Acid etch it again and spray some primer on it without even touching it.
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[Thrasher22]

Really? Good to know! Maybe I took their "wipe off excess" a little to far.
I gave it a second coat last night, so I'll follow your advice when I get home.

[Thrasher22]

After I rinsed the part off last night, the discoloration came back almost immediately... could it just be discoloration, I can't imagine it would rust that quickly.

[JerryMCarter1]

Yes it will rust that quickly -- there is a lot of oxygen molecule in the air.
I usually acid etch then water wash - then air dry - then prime--within mins - but I always us a sealer after primer immediately or within a very short time either sealer or paint -

Primer is water molecule pourus
Have no fear it has been done this way for years - one can also walnut blast -- but what a mess
Jerry
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[tspot100]

thats why ospho is left on after it dries. no need to wash off. no water.. no rust! applied too thick/heavy and you will have problems. the whitish powdery residue can be brushed off and apply epoxy primer then topcoat. the ospho must be dry before topcoating...

[beetlenut]

Are you sure it was phosphoric acid and not another kind of acid, like hydrochloric acid? It should't flash rust with phosphoric acid, but would in hours with hydrochloric acid!
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[JerryMCarter1]

Acid is acid -- remove it before primer sealer
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[Thrasher22]

[nathansnathan]

Phosphoric acid is a rust 'converter' in that it will chemically transform reddish-brown iron(III) oxide (Fe2O3 - rust) to black ferric phosphate (FePO4). What remains when you use metal ready or like ppg's dx520 is that as well as the golden zinc phosphate.

I have read some say that to let the metal form a layer of surface oxidation and then use phosphoric acid to 'convert' it, that is a good base for paint/ primer. Make sure though, to get all the excess phosphate/ zinc off though as this is the #1 reason for paint not sticking I'd read over at the ppg site. The excess, I think is often caused by people not keeping the piece wet - you're not supposed to let it dry before you rinse it off.

If it is coming out blotchy, though, it's on account of poor prep. You have to make sure you've been consistent, or it will blotch up the spots you've not hit as good.

[banditwolf]

I have been using Fast Etch and have had the best luck with putting a layer of plastic wrap over it and smoothing it out. Doesn't air dry. Then wipe it off as suggested with Acetone.
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[Goshen]

I used the same product with great results but dont immerse the piece in it.. just brush it on... look at the pics below:

I first sanded and removed rust with a steel bristle wheel on my high speed die grinder that i bought from Harbor Freight tools for $12.00

I used a product that I bought at ACE Hardware called Ospho but you have to be careful handling it since it is Phosphoric Acid and can burn skin.. after letting it dry 24 hours for the entire floor of my '67 Bug. I used POR15 from Eastwood.

After that was all dry Chris Vallone from ClassicVW Bug suggested a deadening material called Quick Roof which i bought at Home Depot for $16.00 for a 25 foot roll.

Just look at the photos so you can see the process:

STEP 1 BEFORE OSPHO:

STEP 2 AFTER OSPHO:

STEP 3 AFTER POR 15:

STEP 4 QUICK ROOF DEADENING MATERIAL:

[img]http://i202.photobucket.com/albums/aa300/zechrah/2003%20MERCEDES%20BENZ%20C240/MY%201967%20VW%20BUG%20PROJECT%20%202012/QUICKROOFDEADENINGMATERIAL1.jpg[/img

This is a good Seam Sealer that you can buy at Home Depot for less than $5 bucks and it's paintable:

[Thrasher22]

Thanks for all the replies! The blotchyness wasn't a big concern, anywhere it looked like that didn't have any rust when I picked up the part.

After 3 rounds of acid there were a couple spots that still weren't fully converting, so I just hit it with a thick coat of POR 15, and will spray the inside of the pillar with some Fluid Film to protect the welds as best I can.

[banditwolf]

Ok. I have only read the instructions. I haven't gotten around to trying it yet. Sounds like I don't need to worry too much when I do.
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[gfw1985]

Not a VW. I've used phosphoric acid quite a bit and have never rinsed it off. This one including tranny/transfer case was treated using a foam 1" roller, dried 48 hours, rust incapsulator for primer, and shot with Chassis Black. Also done many body panels, but 2K primed and top coated with single stage acrylic enamel. Not to say you won't have problems, but it has never failed for me. Still looks the same three years later.

[Goshen]

[Thrasher22]

[Big Papi]

I'm no pro but you gotta go phosphic acid followed by an acetone wipe down. Washing w/ water allows flash rust to form no way to avoid it. Leavin the phospho on creates this leftover "salt" precipitate to form. that can't be very good for paint adhesion, so its gotta come off. acetone is hydrophobic and is so volatile that it evaporates as you apply it. good stuff
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[nathansnathan]

I would argue that you can rinse the stuff with water. The test ppg gives to determine whether or not you have rinsed the stuff off is whether water will wash over it in sheets or whether it will bead. Also, their stuff, you have to dilute it 1:2 with water to begin with. As the stuff is soluble with water, I don't see using acetone.

Before you put primer down, you would want to wipe with acetone or lacquer thinner then, but for rinsing off the water-soluble product, I would use water.

[gfw1985]

I have never used PPG's product. When using any product, I follow their directions and am the last person to tell you do different. Just about any paint manufacturer "cautions" about using phosphoric acid, telling you to test in an inconspicuous place. I use Jasco from Lowes and so far have never had a problem with the paints I have used. I am not a professional painter and do not pretend to be one. I do wipe down with grease and wax remover whenever I paint anything. Only time I ever use water is color sanding before buffing.