Hi Mike
What glue did you use to attach the internal fittings? as this might be the problem.

Hi I had a look at the boat you are painting and did not realise it was vac formed. the problem can be caused by the glue bleading through most glues weld abs so react with the plastic if it was only a few days between glueing and painting it was prob because the glue had not had time to finnish “fumming” the solvent evaporating, if it was longer it may be due to it etching on the other side did you sand down the primer befor top coat? if so was there any sign of the etching on the out side ? black is one of the worst colour for showing marks in finnish ask any car painter. anyway you may have to re prime the areas I would use a filler primer and sand it down , most are grey and you may have to prime all the are as if you use a light primer it may show through as lighter, you could also give it a few coats of the red to the efected are and sand in between.let me know how you get on Peter