Preparing an aluminum boat for painting is the same as in the aircraft
industry. Go to your automotive supplier and buy some Aluma-Prep 33 and
Alodine1201. This is an acid etch which will deep clean the surface. Use a
scotch brite pad and thick gloves. Scrub the surface and you will see
black residue coming off the surface. A light amount is what you are
looking for...too much black is called smutting and means the solution is
too strong. Rinse with water and look for the water to sheet off with no
breaks in the surface. Where the water breaks, there is contamination and
the paint will not stick. After the surface dries, sponge on the 1201
Alodine until the surface turns a light gold. Now you are ready to prime
and paint. I suggest you use products by Axon Aerospace
www.axonaerospace.com. Here is a link describing the procedure used on
aircraft. http://axonaerospace.com/EP2Y1standard1.html

If you don't follow these standards, you are susceptable to filliform
corrosion and paint adheasion failure.