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Old 05-14-2009, 07:02 PM
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What makes paint bubble up?

I'm a beginner so please bear with me. I'm wondering what makes paint bubble/wrinkle. While prepping my truck, I was told to remove all traces of the rattle can primer that the previous owner had applied or risk the new paint going over it. When I was a kid, I remember spraying lacquer over enamel rattle can with horrible results (I think that was what I did).

Anyway, I can understand about chemical incompatibilities, etc. but when I discovered that most base coats are not mixed with an activator, this got me wondering why it does not bubble when you spray clear over it. I recall reading somewhere that lacquer is just pigment dissolved in solvent and that you could, in theory, scrape off dried lacquer, add thinner, and respray it. Is base coat similar? Can someone explain how or why paint might bubble up?

The reason for the question is sometimes one might want to just spray some rattle can paint on a spot repair to prevent rust, a quick and easy way to do it. But, to have to remove all traces of it later can just be a major pain.

Thanks, Scot

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Old 05-14-2009, 07:15 PM
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rattle can primer will absorb moisture and cause rust you can't see.. best to just leave it alone
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Old 05-14-2009, 07:49 PM
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Like was alluded to, the rattle can primer will absorb moisture, especially if it's been on the vehicle a while before the top coat is shot on. The moisture will do two things -- first, the color will not have a good base to stick to, and second, the moisture will react with the metal and cause iron oxide --- rust. This will cause the primer to peel back from the base metal and everything above the primer will come bubbling off with the primer.

Use 80 grit on a DA, and shoot the panels as soon as possible (within the hour of the panel sanding being completed, especially if you're working outside) with a good etching primer.

The final product will only be as good as the initial work you put into the base. Every little bump, dent, and scratch will show through....so make the base as good as you want the topcoat to look.
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Old 05-14-2009, 08:17 PM
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Oops, sorry...I think I may have worded things unclearly. I was thinking more along the lines of spraying rattle can paint. After it dries and if you apply another type of paint over it, it will likely bubble or wrinkle. Is this always true if you spray automotive paint over it? Why does this happen?

For instance, I have some trim pieces that I'd like to paint black. Rattle can would be quick and easy to get it done now. I'd like to redo it later in "real" auto paint but would hate to have to strip every speck of the rattle can stuff off. The big areas are easy, the little crevices are a lot harder. I know...do it right the first time but sometimes that's not practical.

Thanks!
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Old 05-14-2009, 09:25 PM
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For trim, the easiest way to refinish is to sand the rusted areas down to bare metal and shoot a couple of coats of etching primer. Then use a few coats of what's called 'trim black' for a finish topcoat.

Issues with rattlecan paintjobs are generally (1) improper/insuffucuent prep, (2) paint too light, and (3) paint too thick. The first is easily fixed - clean the area. Any loose metal/paint/dirt/anything else will cause the paint to flake and bubble after a bit. The paint being too thick or thin is completely the responsibility of the shooter. When you're starting, keep the can upright, pointed directly at the surface, and about 8 inches from the surface. You want to have the can moving prior to starting the paint, and have the paint lay down where it's not so dry as to not cover with two or three coats, but you also don't want it so wet that the paint is dripping off.


When it comes to colors...the color/clear/primer must all be compatible. But the basics still apply -- cleanliness and proper technique. Learn to shoot with a rattlecan; there's nothing wrong with the paint and it's cheap. In my opinion, there are so many bad rattlecan jobs out there that every rattlecan job is carrying that bad name. But do your homework, use compatible products, and keep the worksurface clean if you want your work to last.
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Old 05-15-2009, 07:59 AM
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The "problem" is that most rattle can is enamel and it never really dries completely,so when you come back later and spray more rattle can or automotive type paint,the SOLVENT attacks it and that is what causes the wrinkling. You can re coat LACQUER rattle can over and over itself but enamel is best done right the first go around.
Todays automotive paints use a system of chemicals DESIGNED to be re coated BUT,even they will pop or wrinkle if re coated too soon.It's ALL about SOLVENTS and letting them escape before the next coat,other wise,you'll have the SAME issues.
Nothing "wrong" with doing interior pieces with spray cans but exterior needs the right paint.
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Old 05-15-2009, 08:06 AM
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"Wrinkling" is ALL about solvent from the next product getting UNDER the substrate product.

This can happan as Bee4me said by the solvent soaking thru, OR it can happen by the solvent getting under the edge of the substrate product.

It can happen to 2K as easily as 1K products. If you spot primed an area over OEM paint on a car that wasn't scuffed well, even with 2K primer, when you paint over the whole panel the solvents from the paint can get under the edge of the primer because it doesn't have proper adhesion to the poorly scuffed OEM paint.

OR if you have too thin of a film of a 2K from lack of application or from being sanded thin, it too can "lift" or "Wrinkle".

With 1k rattle cans all bets are off as mentioned before, the solvents just jet right on thru it and wrinkle city.

Brian
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Old 05-19-2009, 07:06 AM
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Thanks for the replies, great info! A couple more questions...

If one were to paint something with intentions of later repainting with automotive paint, is lacquer the safest bet for rattle can stuff? Enamel sounds like it's definately out of the question. I should clarify that I would remove as much of the rattle can paint before recoating, but might not get every bit of it off.

Out of curiosity, is urethane reducer more or less aggresive/harsh of a solvent compared to lacquer thinner?
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