Trying to remove old paint from a tin boat can be a long, laborious process, that can kill your enthusiasm for the project quickly. To do it quickly and easily, with little or no effort, and no damage to the aluminum surface, go to the Internet and look for 'sodablasting' in your area. This is a process similar to sand blasting, but the stripping medium is coarse bicarbonate of soda. It will quickly take your boat down to bare metal, and the used soda will dissolve in the next rain, eliminating most of the mess. The average tin boat can be stripped for about $100 if you use a commercial stripper, and much less if you already have a sand blasting rig. This process is used to remove the paint from the fiberglass bodies of Corvettes, so they can be repainted, so it is a proved process. Check it out. It just may be your savior.