Chinese filler - designed to meet just two criteria - quick to dry, easy to sand. So that production can run at the most rapid pace possible.
Back in the day when new car tires (Bridgestone?) were failing, I'd read a snarky comment that the car manufacturers had just two criteria for new car tires - round and black (and the "round" was negotiable)
Back in the day when new car tires (Bridgestone?) were failing, I'd read a snarky comment that the car manufacturers had just two criteria for new car tires - round and black (and the "round" was negotiable)