Cleaning and prep is crucial for creating a good bond. We use a solvent based, spray-only lacquer, so we have a specific set of cleaning instructions. Our lacquer bonds well with fibreglass or the gel coat on the fibreglass and also on Bondo when used.
Read the instructions and ask questions wherever you buy paint
Spider cracks will continue to grow if you don't deal with them even when filled with Bondo or fibreglass. They have to be cut out. A Dremel tool can be used to 'router' each crack line past its present length until you are in stable territory. Then fill it. A light
Scuff-Sanding and cleaning, removing all caulking, etc.
Then follow paint
And .... good luck.
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The Little Camper Painter