Question: You say you would not paint aluminum. Ok, it is hideously expensive (I have been quoted £10,000 just for materials), it is difficult to key, and we have been advised to sand blast and paint the same day (tricky!). So, if you were to start all over again with a newly built boat, what parts would you paint?
Answer: You are right, painting an aluminum boat is incredibly expensive and if you get someone else to do it the materials price you were quoted will be less than half of the final cost.
Yes, you must key the aluminum, but there is much more to it than that. You also need to acid wash and get a good zinc chromate primer on that day (this goes under the normal high build primer). Don’t believe anyone who tells you that acid etching and metal primers are not necessary. Even some professional painters will try and tell you this because they don’t want to deal with these nasty chemicals. If you don’t etch and prime properly, all your expensive paint will start bubbling in a couple of years. By the way, the same applies to painting aluminum spars.
The only part of a new aluminum boat I would paint is the bottom. Starting with a primer as discussed above, followed by a good barrier coat (we use InterProtect from Interlux) and topped off with an aluminum-friendly anti-fouling (we use E-Paint ZO).
Definitely do not paint the bilge. You will never get the paint to stick properly and you will then have to live with loose paint chips in the bilge with the potential to block bilge pumps, block limber holes and gather in wet clumps with any dirt. This will actually promote corrosion rather than prevent it.