I'm painting a replacement hatch cover that goes over the cabin door on my 38 FBDC. I'm going to roll and tip it.

Just picked up the paint and read the instructions while at the store, it says to use 'Activator'.

Paint $68 a Qt
Activator $57 pint

Question is, what is the activator for? Do you have to use it? Is it a thinner, hardner?

Thanks in advance, Rick