This stuff right here is friggin' magical.

http://www.amazon.com/U-S-CHEMICAL-P...4916004&sr=1-2

http://www.amazon.com/USC-Icing-24-t...4916004&sr=1-1

It's a two part system so it's a bit like a resin. I believe it's urethane based and it's extremely flexible. I've used it several times on prototypes that were cast for toy productions, etc. I also used it on my old amigo with a fiberglass filler underneath.

Depending on your crack you should take a very small very fast drill bit or end mill and drill a tiny hole right at the end of the crack. If you aren't sure of the actual end of the crack, drill just a fraction of an inch out from it. That will provide relief in the form of a curved surface and stop the crack from continuing. Once you've done that you will want to groove the crack a little so you have some depth to apply your filling solution. The stuff I linked to is VERY low shrinkage, so you can put in one coat as the filler for the crack, then go back with a very smooth skim coat.

I'm betting you'll be good for a while if not permanently with that.

Then like the others said, that 3M dinoc is amazing. It's what they are using to do these "stealth black" "paint jobs" on these insanely expensive super cars. It's guaranteed for like 3 years or more in direct sunlight if I'm not mistaken. If 3M has taught me anything though, it's that their product typically FAR outlast their "guaranteed" lifespans.

It's also amazing stuff to work with. Just heat it on the edges and wraps with a heat gun and it sucks and smooths down to the surface like magic. It's a pleasure to work with.