no, i'm no expert in paint, but i don't know whats going on here....

got second hand panels for my pickup, fibreglass/kevlar panels, don't know what resin has been used, and most of them (certainly the one's i've painted so far are off the same car)

the paint alreadt on there comes of with cellulose thiners, so i assume is cellulose paint. sprayed 3 panels so far. no problem as all (except i didn't sand them smooth properly before painting. no worries, just wanting to make them more presentable for now)

used plastic filler primer (halfords aerosol can. also comes off with cellulose thinner) on them so far, then sprayed in cellulose paint. looks alright.

now this panel i'm trying to do not, original paint comes off with thinners, left it on mostly (as i did with the other panels), sanded it smooth(er) as it was full of pits, and sprayed with plastic filler primer, it keeps bubbling up and peeling off! why would this be?
i rubbed it down, wiped it over with thinners to clean it, sprayed again, and its peeling again!

whats wrong?

Use much lighter coats, for a start I think you’re spraying too much on at a time and the thinners aren’t getting enough time to evaporate fully. Filler primer is so thick it takes ages to dry.

You wiping it constantly with thinners aren’t helping either as the GRP is porous and is soaking it up like a sponge, don't do that at all, cellulose thinners can actually permanently damage GRP and make it swell. If you have to use thinners use white spirit but dry of with a hairdryer, I just don't use anything but water with a bit of fairy liquid.

Sand it back using no more than 400 wet & dry then get a hair dryer and properly heat the panel up to boil off the thinners. Then using light dust coats prime the surface. Take your time then leave it a full day before applying the colour coats, lightly. Buy some latex type gloves and use them when handling the panels, if you touch it without them, wash and dry it again. Btw paint and lacquer are the best way to use halford paints, I did it this was for my old motor bike and the finish is amazing. Put on a few colour coats, then sand that lightly with 1400 grade then once perfect, several light coats of lacquer and perfection.

If you buy this months 'Practical classics' car mag there is a really good section on spray can painting, well worth a buying it just for that.

[Edited on 1/10/08 by Mr Whippy]

Sometimes there is just something in the old paint that reacts with the new. Solutions are strip the bare surface and etch/prime/topcoat or use an isolator/sealer like Barcoat. Buy here?

Acrylic paint will also come off with cellulose thinners and bubble when spray on top off.

As has been said, just because cellulose will take it off doesn't mean it is cellulose - in fact nearly all modern aerosols aren't.
You can try sanding off all of what's there, if you don't get it all off it will still react. Unfortunately once you've disturbed the sub layers it can take months to become "dormant" again.
Best way is to sand flat & seal with something like barcoat as mentioned (there are other alcohol based sealers around), if you don't want to do that then get it really hot for an hour or 2 b4 spraying & use really light coats - if it stays down don't be tempted to put "just one heavier one on" to get a shine as this will probably make it all pickle up again - you're better to spend a bit of time 12/1500 flatting & polishing up afterwards.

cheers guys i think its some weird incompatability paint issue, weid it doesn't happen al over though. i have been using light coats, and leaving it a day to dry before sanding, or wiping with thinners and stuff. weird.
its going to be in places that won't be seen anyway. also, as said before, for now i'm just tarting it up and making it all one colour. i'll probably re do it properly next winter. for now its all about getting it SVA's before sva ends