What is the cause of paint fry? I have had it when applying primer over a base coat and vice versa and also have had it when painting 2 colors and 2nd eats into the first.

Are there any general rules to avoid? I have also notice not to apply more paint when it starts as it just grows.

Different types of paint. You are trying to spray a more harsh paint on top of a lighter grade paint which is acting as paint stripper. Rules of painting:

Enamel can be sprayed over any other paint as long as the original paint is cured
Lacquer can only be sprayed over another lacquer paint. It will eat into enamel or urethane and bubble the paint.
Urethane can be sprayed over any paint as long as the original paint is cured.

The ONLY exception to the urethane paint rule is the House of Kolor line of paints. They use an extremely harsh mix of chemicals that will bubble other paints if applied over them. They are one of the only manufacturers that allow you to clearcoat with lacquer OVER their urethane without bubbling their paint. They try to make the client use their paint line exclusively.

quote Originally posted by IMSA GT: Different types of paint. You are trying to spray a more harsh paint on top of a lighter grade paint which is acting as paint stripper. Rules of painting: Enamel can be sprayed over any other paint as long as the original paint is cured Lacquer can only be sprayed over another lacquer paint. It will eat into enamel or urethane and bubble the paint. Urethane can be sprayed over any paint as long as the original paint is cured. The ONLY exception to the urethane paint rule is the House of Kolor line of paints. They use an extremely harsh mix of chemicals that will bubble other paints if applied over them. They are one of the only manufacturers that allow you to clearcoat with lacquer OVER their urethane without bubbling their paint. They try to make the client use their paint line exclusively.

Very cool information!
Thanx!

I use lacquer clear at times over any brand of basecoat with no problems at all. I always do it if im painting like a molding or mirror or other smaller panel so I can install it in a 1/2 hour. I use R&M/BASF, Sherwin-Williams, Ditzler, ICI. I think of paints as to 'temps' starting with synthetic enamel being the coolest and lacquer being the hottest. Generally you can put 'cooler' paint over any 'hotter' paint. There are ways to get around it if you have to. There are barrier coats available that will 'seal' a cooler paint to enable hotter paint to be applied over it. One of those is waterborne primer. I also can do it with lacquer primer by 'dusting' on muliple light coats until enamels are covered with an adequate film layer. Basecoat colors in urathane type paint has the same identical characteristics as lacquer paint in that it dries very fast and will accept the 'hotter' lacquer clear. You can wet sand a flaw out of urathane basecoat usually within 15 mins or so of spraying. The urathane clear however needs at least a few hours to set up and overnite to dry enough to sand. Lacquer clear can be sanded usually within a 1/2 hour to repair a flaw. Although it can be buffed in a few hours, you preferrably should wait overnite. Using lacquer clear for those smaller parts, I can sand and paint the part, install it, polish it and deliver the car almost while customer waits.

quote Originally posted by rogergarrison: ..... I also can do it with lacquer primer by 'dusting' on muliple light coats until enamels are covered with an adequate film layer.....

This is true. I didn't mention it because I don't know Australian's paint expertise

I wasnt attacking you, just pointing out possibilites Ive learned thru experience. I DO realize most in here are complete novices while others assume everyone here has a degree in auto body repair and refinishing lol, and unlimited financial resources.

quote Originally posted by rogergarrison: . . . and unlimited financial resources.

Isn't that mandatory, Roger? You can't restore a car on a budget.

Its allways a good Idea to park the vehicle in the shade for a good 24 to 48 hours to cool the body,the engine should allso be cool to prevent heat transfer.Painting should be done in the shade.I remember that there is a wait procedure,when applying a 2nd coat,too soon can cause an orange peal like condition.Try Googleing Automotive Painting Tips.

Undoubtedly....DONT paint in direct sun. Panel will be too hot and will end up feeling like 80 grit sandpaper. The one exception would be if i was trying to get laquer primer over plain enamel. It would help that it dries before it gets a chance to soak into the enamel and bubble it up. I use a heat lamp instead of sun though.

Well i had a lot of problems with over spray enamel from old rattle can job among many other things. I have dusted clear on before but i am far from a paint expert a couple of cars doesn't make make me an expert just competent. This time around though i am using all the same brands i was using different brand of hardeners colors thinners and sticking with debeer now all the way due to issues.

Nearly all rattle cans are just straight enamel (coldest paint) and nothing else goes over it without problems. I always say its far better to remove spray can paint however you can before you do anything else. I even tell anyone NEVER to cheap out and use spray cans to cover it till they get around to a real paint job...its far more work than to leave it alone.