I dont know for sure but i would suspect that maule uses steel or steel stem rivets and you are seeing corrosion due to dissimilar corrosion. I know it is a very common sight on mauls or other airplanes with steel rivets on alum structure.
Obviously a full strip with a mix of grinding, sandblasting, rivet replacement and even re-skin would yield the nicest results. This would be over kill for most people and especially if you want to fly this summer. Maximum acceptable thickness loss of aluminum skin is usually 10%. Chromate anything primer is junk. If you are going to repaint use a good epoxy primer.
Spot blasters work well. They have a containment dome to keep the sand from going into your eyes and ears. Blast, etch alodine, prime, paint.
http://www.amazon.com/Aircraft-Tool-Supply-Rivet-Sandblaster/dp/B005VR84TG/ref=sr_1_6?ie=UTF8&qid=1405213147&sr=8-6&keywords=spot+blasterYou could use a die grinder with a scotch brite wheel.
You could drill out the rivet blast it, grind it and paint it replace the rivet.
With all that said unless you think the structure is being compromised or the rivet head is corroding off i would be inclined to leave it alone. Otherwise you are going to wind up with a million little spot paint repairs unless you chose to repaint the whole thing.