The Stovebolt Site Forums The Shop Area Paint and Body Shop Questions about Phosphoric Acid (with pics)

I did a test yesterday using Phosphoric acid to prep some parts for primer and since I've never done this before I didn't know what to expect and I have questions about the results.

The instructions on the container list two ways to use it.

In this FIRST TEST I applied the acid full strength and let it sit overnight according to the instructions. As you can see, this part has some deep rust pits. This morning I rinsed the part with water and sanded a portion of the treated area. Am I to assume that the gray material left down in the pits is an inert material that can be primed over without any further treatment other than a wipe down with grease and wax remover?

In this SECOND TEST I applied a diluted solution (2:1) to sandblasted clean parts, let it sit for 20 minutes, rinsed it with water and hung it up to dry, and this is what have this morning. Is this right? Can I primer right over this? It sure looks like rust to me.

Any input is greatly appreciated.

Thank you.

first one looks good, start painting soonest ... 2nd one > you needed to leave it a lot longer than 20 min when diluted, overnite may have done as well as the 1st, do it over

Bill

Woody, the metal prep is etching the surface and you will have a matt finish gray texture after letting it work. Straight metal prep should not have to work over night. In fact you don’t want it to dry out so you would have to submerge if left that long. Normally you would only treat the surface with the metal prep, just keeping it damp and scrubbing it with steel wool or a brush, shouldn't take more than 5-15 min than wipe it dry. If you also picked up a metal conditioner that’s good, use it right after the metal prep has been wiped dry and you will have some protection till you can get it primed. Just keep in mind that after the metal is etched it is extremely subject to oxidation and will begin to scale over in a very short time, a matter of minuets if the humidity is high. Using the metal conditioner right after etching will give you a little more time. I have kept bare steel for several days with out seeing any signs of rust after treating it this way.
A good epoxy primer after treating the metal will give you a base for your build up and finishing work that should last as long as you will with out any problems.
Denny Graham
Sandwich, IL

Phosphoric acid is pretty tame as acids go. I would use it full strength. I wouldn't trust the residue at the bottom of the pits to be "inert". If it were my truck I would do as you have, use the acid full strength, then I would treat with a rust converter to treat what's at the bottom of the pits. Don't ever let bare metal sit over night or you'll get what you have with your second pic (or worse). Depending on the humidity it can only take minutes for rust to start forming on unprotected steel.

Sandblasting the parts and treating them with the acid is great but what went wrong with the second test is you diluted the acid, presumably with water, then rinsed the part with more water and hung it up to dry. Water, oxygen and metal = RUST. Do not prime those parts as they are. Do them over with the full strength acid.

We used phosphoric acid extensively for metal prep while I was going to sea. I assume it is still used widely. When prepping badly rusted areas the rust (iron oxide) was either chipped, or wire-wheeled prior to treatment with phosphoric acid. Application of the phosphoric acid converts the iron oxide to iron phosphate which is the inert or protective coating. The manufacturer's instructions stated that you could paint directly over the iron phosphate without rinsing with fresh water. Generally, that practice was followed unless there was a fairly substantial coating of "loose" phosphate powder on the surface.

It all sounds great but in over 30 years of going to sea we never found a product that would provide long term protection of the bilge or tank top areas in the engine room. We tried most everything from the exotic 2 part expoxies, requiring that the crew members wear clean socks only on the prepped metal, proper drying time, attention to temperature, application thicknesses, etc. Nothing seemed to work for a period greater than 3 years. Epoxies (up to seven coats)worked fine in salt water ballast tanks, that were only exposed to air for about 1/2 of their life cycle, but the metal was sand blasted to near white metal SP-10 prior to exacting application standards.

If you want to learn about metal coatings and rust prevention Google something like "Ship protective coating rust". There are new coatings coming to the market as fast as ships rust away, but it all boils down to preparation.

Oh - to answer your question... I agree with the others-don't sand the treated area, as you will remove the iron phosphate protective coating, and do use the acid full strength.

first one looks good i think you would get the same from the secound if you leave it over night thats what i do when i spray on ospho you have got some clean metal in your first picture

Thanks everyone... this issue has been resolved. Use full strength, let it sit overnight, remove any scale and prime.

BINGO... exactly what happened.

It doesn't matter if you agree or disagree. Point is that's exactly what I did and that is exactly the results.

However, you are correct that the instructions say you can do it that way. Unfortunately those are results you can expect by following the instructions.

My votes still for electrolysis first.

Tell you what... I'll send you the door and you can find a vat big enough to hold 60 gallons and a door.

I just don't have the space nor the inclination to go through that but I do admit that it would probably be the best way to go.

I just re-read what I wrote and that didn't quite came out the way I intended which was to agree with you... in a round-about way. My apologies.

Not to keep beating a dead horse but the problem with the second test may not have been so much the dilution with water part but the rinse with water and hang up to dry part. I'm sure we all know if you have clean bare metal and apply water to it you're going to get flash rust. This is why we all preach priming bare metal as soon as possible and not even touching it with your bare hands as the oil in your skin can cause rust to happen.

So would the procedure be to spray the metal with the acid, let sit overnight, wipe down with a wax and grease remover and then prime? I'm thinking of doing the inside of my cab this way.

That's the basic idea, as long as you are dealing with rust free metal. In my case, I'll knock down any remaining surface irregularities with sandpaper before priming.

I used an inexpensive bristle brush, but a spray bottle is also suggested.

I go through this process when prepping for paint.
1. Wash with degreaser and scrub brush and rinse with water.
2. Wire wheel or media blast the part
3. Immediately spray with 100% Phosphoric Acid (I get it from Home Depot) and let it sit for a few minutes. Wipe off with a dry towel. If there is still some rust repeat until it is gone or has turned sufficiently dark grey. I never wash the parts with water again, guaranteed flash rust. Let part completely dry.
4. Scuff off any white residue that might remain. Run an exacto knife in small crevasses.
5. Dampen (not dripping wet) a cloth with 100% acid and wipe part down.
6. Wipe part down with PRE or Lacquer Thinner.
7. Paint with epoxy primer asap.
8. Top coat when time permits.

I have never soaked a part overnight in this stuff, though it says you can do that. Don't let the acid dry on the part, that is when you get the white powdery stuff which I am sure isn't good to paint over. I have done this process with some great success. It is tedious but preparation is 95% of the job.

According to my results, I'd have to say that jomichael has the process down to a science. I brushed the same thing he's using (from Home Depot) according to the instructions, let it sit overnight (according to the instructions) and I have a royal fkn mess on my hands today.

The surface that was pitted is so rough it will have to either be sanded or blasted again. Some places the surface is gooey... Some places are okay.

Brushing or spraying this stuff on and leaving it sit overnight is a BIG mistake. Either wipe it on or spray/brush and wipe it off leaving a thin film.

Czechman,
Just spray it again with the 100% acid, and wipe it dry. That will fix that mess, I hope. That is the main point with this stuff, wipe off until dry, don't let it air dry. Keep changing cloth too.
Also, read your other post about Eastwoods, don't go through the hell I did.
Michael

Thanks Michael, I sure hope that's all it takes. I'm getting pretty frustrated... BUT!... I'm learning a lot and pressing on. I have a long road ahead of me and too heavily invested to quit now.

these are the kind of things I love about stovebolt, real world experience and steps that I can follow and do on my own project!
Thanks guys