flexible fiberglass resin?

I'm thinking about making making a new bumper out of fiberglass and would like it to be flexible like hiflex bumpers. What type of resin would I need to get that kind of flexibility? Is it just polyester resin or something special?

the hiflex bumpers have very little resin in them, mostly soft plastic and the amount of resin gives the bumper some ridgedness to it so its not flapping in the wind

Ok, thanks. So i guess duplicating that level of flexibility would be impossible on a diy basis. Is there any resin that would add some flexibility so it doesn't crack if it gets hit with a good size rock or something? I read that adding acetone adds flexibility, has anyone experimented with this?