Well, Bob pretty much summed it up.
But I have found that it can alter the color a bit also ( makes it go a shade dark for some reason) I havent used it in years as it makes your dry time slow way down, plus it only allows it to flex to a certain degree , then it will still crack!

Most Paints today have a pretty good flexability as the hardeners in todays paint have been perfected to a higher level, compared to the Synthetic & Acrylic Enamels of yesteryear.

Fiberglass is solid enough not to require a Flex additive in the paint! As long as you use a GOOD paint product.

Man where do guys come up with these Ideas? ............LOL
I have came across some really scarry stuff in my painting days, because of some one giving bad advice to someone trying to do a paint job and it not being their profession.

SprayTech