The best person to ask re: applications etc of poly / acrylic paints would be Jarrod.. Jarrod is far more experienced and knows a heck of a lot more than i do about these things..
But i do know that leaving your axe in the sun will basically destroy it!

Back in my art school days we learned that crackle finishes can be achieved by painting Lean over Fat.
What this means is a thin coat applied over a thicker coat after the thicker coat has skinned over.

As the thin coat dries faster, as the underlying thicker coat dries and shrinks slightly it casues the thin coat to crack.