Performing welding repair on an aluminum boat is a great way to save the craft. As long as you have a welder that is capable of at least 200 amps, along with a spool gun loaded with welding wire and the necessary safety equipment, such as a welding hood and thick leather gloves, the job shouldn't be too difficult.
Clean the aluminum to be welded. Although it will not technically rust, aluminum readily forms oxides on its surface that will prevent it from being welded successfully if present. Score the area well with a wire brush or a wire wheel drill attachment. Not only must this be done to the damaged area, but also any areas where the ground clamp to the welder will be placed. An oxide-free ground location will help provide a strong, clean current for a quality weld.
Preheat the material if it is more than 3/16 of an inch thick. Welding aluminum requires a huge amount of amperage because the alloy displaces heat so quickly. Because of this, the aluminum boat will require a preheat with a torch if the area to be welded is thick. Performing a preheat will take the aluminum up to a temperature that will allow the welder to gain better penetration, which means a much stronger weld. If the repair is minor or the area you are welding on is not very thick, no preheat is required and you can simply weld the area normally.
Weld the aluminum. Now that the aluminum is prepped and ready, go ahead and strike an arc. It is best to weld in a zigzag pattern and move very slowly to ensure both good penetration and also that the weld puddle is dispersed evenly on all sides of the damaged area. The thicker the aluminum, the slower you should move in order to guarantee a quality weld.
Wait until the aluminum cools before any other modifications are made. Because the welded area will, in essence, be just like the original material, it can be ground down and painted or polished just like any other aluminum area. If the welded portion is not in a visible area or appearance is not important, the weld does not have to be ground at all.