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TR2/3/3A Flash Rust

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joshsibille

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I've started the body work on the TR3. I'm stripping back to bare metal. My first project is the passenger door. I've been sanding the old paint/primer/bondo off. So far the door is in good shape with minimal bondo(what looks like an appropriate amount for well done body work. My problem is that I'm working in the evening after work and its taking me a while for each part. In the meantime the South Carolina humidity is flash rusting the sheet metal. I'm planning on taking this one panel at a time(at least to the primer stage). Is there something I can do to stop the flash rust? Or should I just leave it and sand it off right before I prime it?
 
I always keep a few cans of grey primer around. We don't get rust in central TX, but when I lived in LA I'd always spray a very light coating after stripping, unless I planned to work it immediately. In humid areas it is also best to only strip what you can work on for a week...and just accept a light rust until you are done beating the panel and can prime it. A 3rd technique I have used is to spray the panels with kerosene, which will prevent the surface rust for several weeks, even after it appears dry. Just remember you will have to use a solvent to remove the kerosene film before priming.
 
You don't want to put any oil based finish on any bare metal you plan on painting! Ospho is the brand name of a phosphoric acid sold at WalMart and many box stores. More affordable is Milkstone remover sold at Tractor Supply-it has a high percentage of phosphoric acid. Wear gloves and eye protection and apply with a sponge or rag. All rust will dissolve and a protective film will protect the metal for a long time.
 
thanks for the advice. How do you treat the metal after phosphoric acid when it comes time to prime? I'm assuming you can't just slap the primer down over the metal without doing something to get rid of the residue...
 
Most acids you will have to neutralize. Most often with water. In the past when I use metal etching products and neutralized it and before I could get it to dry it would start to flash rust. I wouldn't put any kind of oil on it and especially WD40 which has silicone in it and is a painters nightmare. I use a self etching primer in a aerosol can after doing metal work. You can leave it where you need to and continue working where you need to and re-coating should not be a problem of it etching in with the dried primer. Most epoxy primers now a days have rust inhibitors in them and will neutralize any rust. There is always rust that you can't see. Also now paint companies recommend direct to metal epoxy primers for the best adhesion and sealing then do your "bondo" work over top that. So if you can strip your panel have epoxy primer ready to coat it then do your filling on top after it cures. If you need to do any welding then take my first suggestion. Hope this helps.
 
+1 on Phosphoric acid wash. Let it dry then rinse it off with a little soap and water. The application should protect it even after washing and drying. Before you go whole hog, try a piece. I've left phos on wet overnight for tough surface rust and it removes it completely (like a miracle!), then wash - especially the crevices that might capture some. You could probably even wait days after yours' flashes and then do the phos. Try it first since SC is high humidity and Maine (where I live) is generally not. hth

Home Depot sells square gallon cans in the paint dept.
 
I have never had luck with acids...mostly with the “neutralizing” part. Whenever I have used them I’ve done more damage than help to the metal. I’m sure I was missing something, but in the end I found alternatives I liked better.
 
I'm currently stripping my TR4A tub. I do only the amount I can finish in a day, then clean it and prime with epoxy primer. It's a bit of a nuisance to have to mix paint repeatedly, but I find it preferable to the alternatives.

I am also skeptical of putting anything on the surface that has to be cleaned off. You have to be SURE that you get it all off, and that may be difficult.

I have experimented a bit with phosphoric acid--just the cheap, big bottle, nothing fancy. I understand the chemistry, but don't understand why it just didn't work for me. I got a good growth of iron phosphate on the surface, as indicated by the dark gray color, but a very light yellow coating, which I assumed to be rust, still formed in a few minutes. Apparently, some of the iron on the surface didn't get converted, but I am not sure why.

If you search the internet, you will get a lot of opinions about using phosphoric acid, much of it highly suspect, to me. There is a lot about "acids" being left behind, causing some primers to fail but not others. I suspect that, in reality, the users are just not washing the phosphates off completely, and that is simply preventing good adhesion. There are many suggestions about using water to neutralize the acid that is assumed to be on the surface, but water won't neutralize an acid. I suspect that, in reality, the water just washes off remaining unreacted phosphoric acid.
 
Something interesting from the Wikipedia page on phosphate conversion coatings:

"Phosphate coatings are often used to provide corrosion resistance, however, phosphate coatings on their own do not provide this because the coating is porous. Therefore, oil or other sealers are used to achieve corrosion resistance...

Most phosphate coatings serve as a surface preparation for further coating and/or painting, a function it performs effectively with excellent adhesion and electric isolation. The porosity allows the additional materials to seep into the phosphate coating and become mechanically interlocked after drying."


So, it sounds like simple phosphate coating can be porous and allow the light rusting I observed. Perhaps some of the commercial rust removers, based on phosphoric acid, have additives to prevent the porosity. Might be a good reason to use them over the cheap stuff.

Bummer. I like cheap and simple.
 
Any acid whether a acid wash, metal prep or a self etching primer is a big no no under epoxy anything. The 2 are not compatible and may look fine for maybe years then the cracking begins. I've seen it and not pretty. I do work in the body shop industry and run into these problems often.
 
Back when I researched this, I came across similar statements, many every bit as certain. But my problem is, how do you KNOW (not "think") that the acid treatment caused cracking years later? I don't doubt that the cracking occurred, and that the customer used an acid treatment before epoxy priming, but those facts don't prove that the treatment combined with epoxy was the cause. Do you know of a precise mechanism by which the treatment might cause paint failure?

When you treat steel with phosphoric acid, you end up with iron phosphate on the surface. As long as you wash it well, that's all that's left; no acids, as assumed in many of the posts I read. Iron phosphate is known to be a good substrate for many kinds of paint, and I have a hard time believing that modern epoxy primers are that fragile. Are there warnings about this in the primers' instruction sheets? There is none in the sheet for the paint I'm using.

I don't question your experience, but I do think that this idea comes from correlation, not causality, and it then gets amplified by repetition. That happens a lot.
 
Welcome to the world of painting. Best thing on the surface is clean clean and clean some more. As far as what to use under the paint follow the paint manufactures instructions! Hopefully your buying from a major auto paint supplier. It's best to stay with the same manufacture all the way through from the direct to metal primer to the top coat. As far as stripping a panel at a time and having the direct to metal epoxy primer ready is the best way to do it. I've done it that way and also with the etching primer. But the etching primer has to be sanded off before applying the epoxy primer.
I remember doing some training on our plastic bumper repair and after i was done one technician said that's not how he wanted to do it. Ask 100 painters and you'll get 100 opinions. Your doing the right thing by asking questions.
 
Thanks for the input Steve and Got. All these years I thought I was doing something wrong with the acids. Glad to hear it wasn’t just me!
 
Got_All_4. But the etching primer has to be sanded off before applying the epoxy primer. .[/QUOTE said:
Do you really mean sanded off as in ALL OFF? Or just lightly sanded or run over with a scratch pad?
Click to expand...
 
Off means Off. I'm saying a dusting to cover the metal to keep the moisture off the panel until you can get to it. primer is porous and in over time it will adsorb the moisture and start to rust. How many cars do you see running around town with primed and in some cases with bondo showing and rusted?
 
I've never had a problem putting 2 part epoxy primer over 1 part etching primer. Epoxy primer is designed to be inert to the both the finish under and the finish over. I merely sand the etching primer to roughen it. Remember that we had 1 part urethanes many years before the epoxy primers became available. The epoxy is better, but the one part will work just fine.
 
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