One advantage to the filler primer, is that is the same color as the gelcoat so if it was to burn through it would be the same color. Been using mostly epoxy lately though. But $40 a gallon of gelcoat and $5 in micro balloons beats $100 for high build polyester primer

What and where do you get these “micro balloons”. What’s the mixture?

http://fiberglasssupplydepot.com/Phe...oballoons.html
depends on what your purpose of using a “filler primer” is as far as how much to use. Half a pint-one pint per quart of gelcoat, and mix it in after you kick it off for best results. If you are going to make your own filler primers pick up a gun with 2.5 tip

Going to give the Microballons a try. Found them on eBay for 19.99 and free shipping. Already have a 2.5 tip gun. Any other recommendations?

You can tint for your first coat a different color so that when you are sanding and see that color you know that you have went too far and need to apply more before burning through.

Also add the micro after kicking off your gelcoat.