Aluminum is a different beast to paint verses other materials. So absolutely remove all of the paint that is there now. Reason for that is because if some of that paint is bad, flacking or corroding, it makes me wonder about the remaining paint doing the same after you put your effort to repaint. So I would remove all the paint from the hull. You can accomplish that via strippers and manual sanding. Then after all that hard work is complete, you have to use a quality type primer for aluminum surfaces. That means either Zinc Chromate primer first OR some quality type etching primer stated just for aluminum. If you ignore that step, look for the finish pealing down the road. After doing the zinc chromate primer, I would follow that up with a very good epoxy sandable primer on top of the zinc chromate. And here is where your final finish gets its flawless look. If you are simply looking for just a simple painted finish, then do a cursory sanding and move on to the top coat. However, If you are looking for a very nice head-turning finish, block sand the primer until it looks and feels perfect. This is the stage of painting you make the world of difference in the finial look. One thing to understand regardless which direction you choose to go, there is no paint that will cover over any flaw in any primer coat ever. So if you can feel a pit, divot, gouge or flaw in the primer, it WILL show up in the final paint... JMHO!
