I use Universal Mold Release from Smooth On for all my molds. I don't have a metal one though so can't answer on that point.
Paws

Morning!
Look up Cresset Aerosol Products.That's what I use here in Canada.You can get it in brush on or as a spray.Not cheap,but a thin coat goes a long way.I've used it for plastic ,metal and urethane molds,no problem.Don't know about fibreglass,should work OK;check the specs on the product line,some are more targeted than others.
Cheers from here