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It depends on what paint is on the car now and if the new paint is compatible with the old stuff..... if in doubt I'd use a good epoxy sealer.
It's so easy to strip several layers of paint with a razor blade, I wouldn't leave old paint on the car.... if it bubbles some time down the road you'll be mad
My 77 project only has one layer, but I'm having a terrible time getting it loose. I tried a heat gun iver the weekend with very poor results. The paint would kind of melt. It would cool and reharden on my razor blade.
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It's so easy to strip several layers of paint with a razor blade, I wouldn't leave old paint on the car.... if it bubbles some time down the road you'll be mad [/QUOTE]
So, what your saying is that the new paint could bubble due to bonding issues with the old paint - correct? Ok, that makes sense.
Not only bonding issues... if for example you spray reduced base coat over old paint you'll never know what that reducer does to the old paint or old primer.... it may soften it, it may create some nasty problems and it may produce gas after you coated that whole mess with clear... then you'll have gas bubbles that are trying to get to the suface but they can't because the clearcoat is sealing it.
You can get away with a good epoxy sealer on the old paint but it's a risk..... better keep scraping and sanding - paint material is not cheap and it's too much work to do it all over again ...
My '80 had been painted and partially repainted some time before I bought it. The red paint had bonding problems in a couple of spots, so I had it media blasted. Plastic media gets the paint off but doesn't damage the glass under it. The media blaster taped all the seams because the edges can get damaged, thus the red stripes.
That's when I learned my car had been in an accident. I spent a lot of time getting my front left fender smooth.
Even though this is a daily driver, I am glad I went down to bare glass and back up again. Surface imperfections would appear as minor distortions with the original color (Code 28 dark blue metallic).