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Go to bare fiberglass. That's the best start for an award winning paint job.
Yep, go to the bare glass. My car had about 5 coats of primer and paint on it when I started. On the rear deck the paint was so badly cracked that when I started with the razor blade it basically was popping off the car in big pieces right down to the glass in some spots. On stubborn areas I used 60 grit on my random orbital sander to get down to the primer and then switched over to 80 grit to get to the bare glass. I stayed away from the edges and hard to get to areas and did those by hand. I think the 80 grit ruffs up the surface of the glass enough so that the primer has something to bite into for better adhesion. I also used 3M sanding sponges that I bought in Lowe's for those curvy hard to get to areas. They come in 60,80,120 etc. just like regular paper. It's definitely alot of work, but it's the only way to do it right.