got some smoothie runningboards
whats the best paint for fibreglass
any other tricks to make it nice

you have to use a flexible plastic primer to avoid cracking because of flex, but the rest of the paint can be anything and will look like metal finish

When using fibreglass, make sure the gelcoat has no cracks in it already from poor handling.
I agree, a plastic prime will enable more flex and help prevent cracking. It all depends on how much the part will flex in the place it is.