Fiberglass is prone to cracking unless it is reinforced.
A much stronger product is Belzona 1111 (Super Metal) with reinforcing
cloth.
I have used this on a Cessna cowling with excellent results. A friend
in Kansas City used it on a Commanche he used to own. The Belzona
repair was still holding up very well when he sold the Commanche.
Belzona will adhere to aluminum.