I have spend a lot of time dealing with thin aircraft aluminum. I don''t know how thick your aluminum is on the mule, but here are several suggestions.
1. Patience. Aluminum is easy to destroy.
2. _NEVER_ use steel or stainless steel wire brushes. They will tiny pieces of steel imbedded in the aluminum, which will corrode later. A wire brush in a drill or grinder will eat your aluminum, and leave a BADLY distorted surface. ( you will understand after you try it)
Scotch - brite at slow speeds is your best corrosion remover.
3. Tal-Strip brand paint stripper (sometimes called "Aircraft Stripper" works well for removing paint, but must be rinsed _THOROUGHLY_. I can find it at my local auto parts store. Use serious chemical resistant gloves, goggles, use it outdoors, etc. It is NASTY STUFF. If you get it on your skin, the burning pain will help you find the spots quickly. Getting it in your eyes will mean a trip to the hospital or worse. Not something you want to wash into your sewers either.
4. A company called Aircraft Spruce ( and others ) sell stuff called Aluma-Prep, and Alodine. The Aluma-Prep is a mild acid that cleanes the aluminum before painting. The Alodine ( which contains Chromate, not something you want to get on you of spill into your sewers ) will make the surface of cleaned aluminum less likely to corrode. With good primer, you may not need it in Canada). If you use this stuff, use it carefully, and follow instructions. Dumping it into your sewer will not make you popular with your local environmental officials. Dispose of waste properly.
5. Aluminum is resistant to _MILD_ Acids. Caustic or basic materials ( found in some paint strippers ) will turn your aluminum into expensive white powder. Engine cleaner and even Simple Green contain enough caustic to start the corrosion process. ( Simple Green now makes an aluminum safe cleaner )
6. If there is an Airport around that services small planes, (or large ones for that matter) Call the repair shop and ask an A&P mechanic for advice. they deal with aluminum and corrosion all the time. An Aircraft paint shop would be a treasure trove of knowledge.
7. Sandblasting: Walnut shells are a wonderful thing if you can find and afford them. otherwise, sand, (or a non-silica abrasive) at a pressure of not more than 50 PSI may work. much higher than 50 PSI will cause aluminum sheet metal to heat from the blasting and become wavy. There is not good fix when that happens. ( Don't ask me how I know ) I suspect even Walnut shells could cause the heating and distortion at high enough pressure. There was a while that some places were using Dry Ice as a blast medium to avoid that problem. Remember Item No. 1 above.
8. Be glad you don't live in Florida where Salt Air and high humidity can really eat through Aluminum.
9. Advice given here is worth exactly what you pay for it. Caveat Emptor.
