Hey there,
I'm bought my 15" aluminum with what looks to have had Gluvit applied on the outside. I have no idea when that might have been, but I since applied it on the inside, paying particular attention to the rivetted areas. Try the outside first and see what happens. If you have "larger" sized cracks or holes, you may also consider using something called "Quick Aluminum" (I think). I comes in a clear plastic tube, and it is an amazing product. I've applied this stuff to holes the size of dimes and quarters on my transom and it seals them up great. Its dead easy to apply and dries in about 15 minutes. It only costs about $15 for a tube. I found this stuff at my local marine shop.
Gman