I did this on the INSIDE of an older aluminum boat I had and it worked great. I had a 4" crack on the bottom that was leaking like a sieve. I sanded the aluminum to clear it of paint, put a fiber glass patch on it and it did the job. It help up for 3 years, then I sold the boat. The advice I got at the time was that this would work if it is in a spot that does not flex too much. Let me know how it works out for you if you try it.