You have to start by getting to the origin of the corrosion. If it's poking out from under the masthead, for instance, you really need to remove the masthead fitting. The next step is to dress the corrosion. This removes the oxide and gets you back to clean metal. The red Scotch-brite Surface Conditioning Discs on a 3M Roloc mandrel are a favorite for this step. These are best used with a small right angle air grinder, but a drill will work OK too, just a little slower. One of those little nylon brushes (the orange one works best) are also excellent. You can find those at home centers, lately. The drill is the perfect driver for a nylon brush.
Whatever you do, NEVER use any kind of wire brush! These always leave microscopic paticles of dissimilar metal behind and cause terrible galvanic corrosion later. Don't do it!
After grinding and smoothing the area down to clean metal, you should etch or acid clean the metal with one of the etchants sold for that purpose like Alumiprep 33 from Henkel, 225S metal cleaner from DuPont or DX533 from PPG. Follow all label or data sheet instructions on dilution. Definitely do not let these products dry on the surface! If the product dries out before you rinse, you have to start over with the etch. Next you will apply a chromate conversion coating like Alodine 1201 from Henkel, 226S from DuPont and DX503 fom PPG. When done properly, these produce a visible change in the color and texture of the metal and leave a gold/straw color. Again, these products, like the cleaners must never be allowed to dry on the surface, or you have to start over with the conversion.
It's all really easy after you get the hang of it. The best way to use these of course is by dipping in a tank of the product, but that's not always practical like for a mast. All these manufacturers have application guides to help you. Awl-Grip's application guide has instructions for conversion coatings as well. They recommend the Henkel products.
Chromate conversion coatings are definitely environmentally hazardous products, ground water pollution being their major danger. For that reason, be sparing/conservative of the product and catch all used product you can. Never dump it on the ground or pour/flush it into any sewer or storm drain! To dispose of it, simply set it aside in an open container and let it evaporate completely leaving a brown solid cake or granules. This can be disposed of cheaply as dry waste, perhaps even for free at your local landfill's hazardous household waste division.
The acid etch cleaners can definitely burn your hands so wear gloves when you are using them. The burns don't hurt right away so it can fool you into thinking it's OK to have your hands in it. The chromate conversion is poisonous (though not terribly so at least in liquid form) so again wear gloves and promptly rinse away any that gets on you. Just use good industrial hygene practices and you'll be fine.
Jimbo
