I am using a MDF mould buck which I need to 'smooth' with primer and top coat of paint - I propose to use a nitro-acrylic paint - will that work with a gel coat and polyester resin which will be used to make the mould?

Thanks

if there is an epoxy surface coat you have nothing to worry about. If it’s epoxy and carbon without a finish then it will corrode over time.

I’d use two part expanding foam with a 4 pound density. I’d avoid the stuff in a spray can.

Jason