The second best time is, as soon as possible. Osmotic blisters, if gone unchecked, can lead to some very expensive repairs. Some bottoms are more prone to blisters, but it's relatively cheap insurance to apply an epoxy barrier coat to all fiberglass boat bottoms.
Before applying antifouling protection, Interlux Interprotect 3000 is what my yard applied to my boat's bottom, shortly after purchase. Check out this link for some good information about preparation -
http://www.yachtpaint.com/usa/