We use cheap lightweight filler but you do get pinholes with it. When I make fiberglass molds (plug portion), Ill hit the big stuff with filler, 2 part glazing putty and then spray with "slick sand". If you are just doing repairs, try spraying slick sand or a urethane primer surfacer without reducer, 1.8 tip and gravity feed gun. Slick sand is essentially a sprayable gazing putty and kicks off pretty quick. You can wet sand and buff either if needed.
I have heard stuff like Rage Gold is much easier to work with than the cheap light weight filler. We just go through too much and don't have the skill level of employees that would benefit from an expensive filler.
Whatever you use, don't let it cure too long or you will have a hard time feathering it out. Use the proper sander for the task too.
