When I stared with(2009-10) SPI, I pretty much just used Euro. Played with different reducers with fast and normal activator. It worked great , other than about 10-15% of the time I would get some die back. It only happened on hoods for cars/trucks. The last 3-4 years , I've swapped out the Euro for Production clear and no more die back issues. I use slow activator pretty much all year, and in winter months of Wisconsin I'll use some polar when needed. I use Production 2100 for everyday collision on bumper covers and fenders. I use Universal(4000) on larger projects, or when I know I won't be cutting and buffing for more than 3 or 4 days after spraying. Speed clear is my new favorite, probably will be used on at least 50% or more of my work now.