I made a few with Alumilite and microballoons. If you useany of these 2 part urethanes (they are all the same) without a filler it will be brrittle and subject to cracking. If you use the microballoon filler, it will be more like a rigid foam and subject to flexing. It takes some getting used to......I had more success by using just a water based clear poly over the paint. The clear poly doesnt get real hard like epoxy and can handle the flexing.
If you are using Featherlite, you may not need to put microballons in it. I think that one is already boyant.
Sonny