best to use a standard base coat primer first mate. it sticks to metal and plastic better. high build primers aren't particullary good at being a base coat to stick whatever your spraying onto. some stay slightly soft as well.
if your going to use filler primer, use it as a second or third coat. get some base primer (like stock grey thinner primer) first. try and use build primer as a filler, as its never a good base and you want your last coat of primer to be relative to that colour, e.g. grey primer with black, red with yellow etc etc.
if you use build primer without a base its a bitch when you sand it as it just comes off in lumps if you use an abrasive material. you got a compressor?
should be able to use the same primer for fibreglass, plastic or metal, due to the fact your spraying on the gel coat rather than the fibreglass resin.