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Black over black and the rust comes back

4K views 27 replies 6 participants last post by  Pats55 
#1 ·
Often the question comes up should I strip the primer off replacement panels. This primer is usually black. It's a difficult question. If the metal had any type of rust and it was primed over at the factory usually in Taiwan it will bite you later.This is a picture of floor pan that was painted over with black moisture cured urethane over the black factory primer about five years ago.The AC drain leaked under the carpet and caused it to rust beneath it.How did that happen , that rust would form under the coatings? The answer ,black is not airtight, the silver should have been used first, then the black.Has anyone else experienced this problem?
 

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#2 ·
This is one of those things without a LOT more info it is pretty much worthless to even think about. If the metal is completely coated in about any automotive primer and paint it likely isn't going to rust. If it is coated with something sub-par like an aerosol paint that is very thin, then it will rust even though being fully coated. But more often than not the coating hasn't much to do with it, because the moisture gets UNDER the coating, at an edge, at a chip, at a worn spot, it gets under the coating and I don't care what you have on it, it will rust.

What exactly happen here, by the photo who knows.

Brian
 
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#4 ·
In the areas where there would be chipping, abrasion or being scratched by fasteners and industrial primer would be required. There are primers that even when scratched the rust won't go beyond the scratch Due to a blood clot I have been working with the body man that has been in the business for 45 years. Some of the systems I showed him and we used he just can't believe.
 
#7 ·
Well there are a lot of people that put black moisture urethane over rust so I initiated this new thread to clarify what happens when you do this. I would like to share some knowledge about other types of industrial coatings that can be useful in prolonging a standard auto body paint job. This is what motivated me to start the Masters series line.If you look at our photo gallery you will see Vintage cars of Fort Mill South Carolina priming everything with the silver primer.they started doing this 20 years ago because cars left outside waiting to get into the shop for years didn't rust with this primer. So there are many options in many different types of paints that a lot of people have never seen or heard of and can be very useful. I didn't start this topic for debate I started it to inform.
 
#9 ·
Sounds like to me your guarding your turf. You can buy industrial coatings in a number of places. Some companies will sell to you, others won't. That's why I usually use generic terms. I know that I shouldn't say the word Mastercoat on your forum. I hope that you and your friends won't gang up on me and hurt me like before. FYI I don't receive any money from mastercoat,I did this to share information.
 
#10 ·
No..I would never gang up on you as you say..:nono:
As far as guarding my turf..That's funny... I just try to follow the rules when at someone esle's house... If you think by ganging up on you because we like SPI,, That's even more funnier.. And if that's what you think this is all about,Your wrong..:nono:

I asked you a simple question...
 
#12 ·
The photo I posted shows the rust under the coating.the rust was caused more by oxygen than moisture. Take a cardboard shirt box and paint the inside with the black moisture cured or the clear. Fill it up with water and it won't leak it's completely waterproof. If you were to take that clear moisture cured and painted it over clean steel within a couple of months you would see rust growing under the coating.
 
#13 · (Edited)
Don't some of them parts come with a E-Coat of them ???

That picture you posted don't even look like someone sanded it before it was primed,,,

Brain hit the nail right on the head.. Who know's how the person did the priming......A lot of things come into play when you put the first coat on bare metal..To me.. It's the most important coat you will be spraying..
 
#14 ·
The floor pans were new replacement pans installed in 2006 they were E coated from the factory and then the black MCU painted over it. Back to the original question should you leave that e coat on them, or should you strip them, acid wash and then prime them?In my opinion don't take chances that there may be rust under that Ecoat especially with body panels.
 
#15 ·
I am with you Pat, but the junk pretend "E-coat" (faux-coat :D) applied over cheap bare mild steel at some overseas plant is a hell of a lot different than REAL "E-coat" Electrophoretic coating that is applied on OEM parts that are even galvanized before the e-coat! Every single part of every single car made the last number of decades is galvanized, fenders, frames, everything!

To tell people to remove E-coat is going WAY over the top in my opinion.

Now, I want to learn something, this interests me, if some cheap primer is over a flash rusted piece of mild steel floor pan and THEN it's fully coated with an epoxy primer, then urethane automotive 2K paint, how does it BEGIN to rust if it is COMPLETELY covered? Let's for the sake of discussion there are no chips or thin spots or anything like that, because THAT is where it going to start. But let's say there is nothing like that. The epoxy primer is a full 3mils and the paint is 4mils, fully applied over the entire piece. How can it rust?

Brian
 
#16 ·
Great question, why would it rust?Oxidation. Most coating systems are not airtight.the latest marketing schemes in the industry is now the hype up airtight. This is the problem they have with the bridges they paint them in a couple years later the rust would come bleeding through.the most effective way to deprive the metal from oxygen is by the use of metallic pigments. Aluminium, stainless steel,mataciois iron oxide.the pigment in multiple layers creates a barrier that the oxygen is blocked.The resin is completely waterproof so you have a moisture proof/airtight seal over the metal.the Bay Bridge is a classic example they always continue to paint. The Pittsburgh Homestead high-level bridge went 14 years with less than 5% corrosion. I hope you can understand the misinformation that a lot of people buy into.
 
#19 ·
The misinformation is that their led to believe that they paint this black urethane over the rust and its sealed forever.That is nonporous.Not exactly true. For many years they were never told how many coats to apply.The name of this coating system is The three coat polyurethane system. It was named that for reason It works better over rust, because the only thing it'll stick to is rust.it's difficult to paint over so it's not really a primer. It doesn't take sunlight so not really a top coat. It's great for a application under a car when used with the correct primer. Brushes out beautifully got an extremely high-gloss and is pretty chip resistant.
 
#24 ·
Stroker I agree you should never paint over rust. You would not believe how many cans of this stuff are sold every day. I stood in a booth for over 20 years and dealt with people who believe this with their heart and soul that when it rerusted it was because THEY did something wrong. Due to DEP sandblasting was prohibited on projects for lead paint that would fall into the waterways or surrounding areas.That's when they developed the silver primer to seal it up It was noted that *sandblasting was a preferred prep but where sandblasting was prohibited'' excellent results were achieved with the use of wire wheels and grinders'.' From there it went to the antique car hobby and was sold intact for a period of time.Then everything changed the resin changed,the thinner was changed and the pigment was changed.The people that had it changed had never touched a car or a can of paint in their lives.to them it was the same stuff. And at least three more companies copied what they did.What and how is sold in the industry and what is sold in the car hobby is different, if this technology is used correctly you will get outstanding results.
 
#25 ·
Rust is the result of an electrolytic reaction. That means there are two different metals, and water, and of course, oxygen from the air.

But you say, there is only one piece of steel here. This is the problem. Steel is iron, and carbon, alloyed together. The rust (oxidation) actually starts on a molecular level, between an iron atom, and a carbon atom. there is enough of a difference in the metals that one atom starts the rust on the next atom, with the carbon atom. Just like sticking a piece of zinc, and another metal into a lemon, and making a battery. An electric current will flow, because of the electrolytic reaction.

Different types of metal have different voltages, if used in the same acid type solution. Once steel starts to rust, the rust is also electrolytically active, and even more so that non rusted steel. There is a higher electrolytic voltage between rusty steel, and plain steel, than there is between the iron molecule and the carbon molecule, used to make steel from iron. So, once rust starts, the rusting is accelerated by the presence of rust.

If you want to read some geeky stuff about iron, and rust, read this.
Iron chemistry
 
#26 ·
Very interesting. I know that a electronic aspect is involved. It's amazing to see 50 years ago we put a man on the moon and we are still dealing with rust.I often wondered why these coatings systems even though they are airtight and moisture proof at the end of 20 to 25 year cycle they begin to fail or have failed pretty badly. Well thank you for bringing up that aspect. I did read the geeky stuff and towards the end I got lost.
 
#27 · (Edited)
I got lost way before the end.

The main point I picked up is that the presence of rust makes the remaining iron rust even faster. Minor points is this there are at least two types of iron oxide, FE2-O3 and FE3-O4.
The FE2-O3 is the red rust, the FE3-O4 is the black rust usually underneath the red rust. The red rust takes more space than the black rust, and once formed, is will lift paint enough to allow more moisture to get under the paint, and now the paint just keeps the steel wet longer, so more rust forms.

I have found phosphoric acid solutions remove the red rust pretty easily, but not so much the black rust. I have had pretty good luck with using electrolysis to remove both types of rust. the process is explained in detail here.
ELECTROLYTIC RUST REMOVAL (ELECTROLYSIS)

Iron rusting is a pain for us working with steel. But in the grand scheme of things, it is the affinity that iron has for oxygen that allows red blood cells to transfer oxygen from our lungs, to our cells, and the CO2 back to our lungs, so we can live.
 
#28 ·
As I said earlier I traveled up and down the East Coast for 20 years and the rustiest place on earth was Webster Florida. I would leave my displays out overnight and the rusty areas that had not rusted in years would have a fresh coat of rust in the morning.It had to be something in the atmosphere.A couple of interesting notes I have never seen the rust creep beyond the edge of the coating and 3 coats sprayed would level all the pitting in sheet metal. Bayer has developed a one coat poly urethane anti-corrosive system is now being used on the Marine Corps Museum.it goes on 6 to 9 mils in one shot. I got to check this out more closely.
 
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