Agree with the above if it want it to last then it will need at least a coat of etch, if its only a few small areas that are busted thru to bare ally thou mate u might find it cheaper to buy it in a can than gettting it in a tin and the thinner for it etc.

If the ally is in good condition and is prepped properly you can actually go wet on wet with etch ie get it all ready then dust a couple coats on then go straight on with a colour after its flashed, but by the sound of it u will need a couple coats of high build on it aswell!!