Cooler first remove the rust what ever it takes sand blasting best, wire brush on drill, usely the navel jelly type products you are throwing money away. Nothing takes the place of good old elbow grease. when you get rust off prime it. And put finish coat on it your local hardware store will have some Rustolem paint or an equivelent. I don't now if it is spot rust or all over but if you can sand blast it. I am fixing to go through the same thing with a trailer, I have the boat sanded down and going to prime it with Zinc oxide primer the only thing that I now of that will prime aluminum. Good luck
MAKE SURE THE PAINT IS FOR METAL.
mikeg