Fillers & Additives

Thickeners, fairing fillers and additives, and which one the job actually wants

Resin straight from the jug runs off vertical surfaces and is too brittle to fill a gap. Fillers fix that, and which powder you reach for changes the result more than most people expect. Everything here is sold from pint and quart jars up to full bags.

Strong, or sandable. Not both

This is the whole decision. Fumed silica (Aerosil, Cab-O-Sil) and cotton flock make a mix that is strong and stays put, which is what you want in a bonding fillet or a structural gap. Cured, fumed silica is brutally hard to sand and will fight you for an afternoon.

Microballoons, Q-Cells and 3M glass bubbles go the other way: light, easy to fair back with a longboard, and not structural. Those belong in a fairing coat over the top, not in the joint underneath.

The usual sequence on a real repair is a structural fill with fumed silica or flock, then a fairing coat of microballoons over it once it has cured. Trying to do both jobs with one powder is what produces either a joint that cracks or a fairing coat that takes a week to sand.

Milled fiber and chopped strand

1/32 in milled fiber adds real tensile strength to a paste, which is what makes it the right choice for a structural bond or a repair that has to carry load. Chopped strand in 1/4 and 1/2 in lengths is for bulking out a deep cavity cheaply before the final laminate goes on.

How much to add

Mix the resin and hardener fully first, then add powder by volume, not weight, until it holds the consistency you want. Fumed silica hits mayonnaise at roughly 1 cup of loose powder per cup of mixed epoxy, microballoons take closer to 1 3/4 cups. Add two thirds, mix it in, then add the rest slowly, because these powders keep thickening for a minute after they look done.

The filler calculator works out the amount for your batch, and the consistency chart is a printable version for the shop wall.