i painted my 81 about 3 years ago and the paint is still as good as the day it was back then except for one spot that is a constant problem. it is the door on the right side. when i sanded the old paint back and was lining up the doors i am not sure but maybe i went a fraction deep trying to line up the door with the body and fear i went through the gel coat. this is just under the door lock section.the paint lasted about 12 months in this section but then small bubbles appeared the size of peas in an area about the size of the palm of you hand. i had it sanded back again and repainted but in another 12 months the bubbles have re-appeared and thats after using more gelcoat the second time on the doorskin before painting. they seem to come back at their worst straight after it rains . i think that water gets inside the door and sweats in there making high humidity therefore causing the blisters on the outside. i dont want to go through the hassle of taking off the door again so my idea was to completely paint the inside of the door for added protection or even spray a 2-pac sealer in there really thick on the inside of the empty door shell. what do you all think is a good solution to stop this from happening again???

 

Did this area have paint defects before your first repaint? This would eliminate some possibilities. Bubbles that appear long after the paint is applied are almost always caused by moisture or other contaminants trapped under the paint film. Water and oils are usually the two culprits. Since it took 12 months for bubbles to appear I'm going with oils, moisture (water) would show up sooner. I have three theories:

1) When you sanded the paint down to the fiberglass you may have ground a small amount of surface contamination into the fiberglass such as oils or silicones that were on the outer surface when you started. Those contaminants remain on the sandpaper and work into each layer you expose as you sand. So it's always important to clean the surface before you sand. Those oils work there way through to the outer surface.

2) Internal contamination caused by oils to lubricate the lock cylinder, lock linkage, window regulator, or door lock assembly. The oil absorbs into the fiberglass and slowly leaches through.

3) Chemical reaction from solvents such as uncured epoxy primer or adhesives that were used for repairs. Often solvents are entrapped due to improper flash times or excessive paint film thickness. Also adhesives and fillers can leech through paint if they don't cure properly.

I would recommend inspecting the inside of the panel, get in there and look with a good light. You may see the problem inside and now you can address it with another question here.

 

the car came from cali and i had it here in austarlia about a year before i painted it. the paint on it was pretty horrible when i got it as it had cracks and chips and peeling paint everywhere but i dont think these blisters were there. i am going with theory 2 that you wrote. i think the fibreglass was reasonably thin due to sanding and lining up the door with the body in the area where the bubbles have apperared so possibly its not very protective. unfortunatly i cant see alot on the inside of the door as the area is covered by the metal subframe or side intrusion bar. i do feel that something is leeching through and maybe the only way to get rid of it is seal the inside or replace that area with some new glass... but then i get worried of it sinking or other problems