The problem with corrosion like that on aluminum is that it goes way deeper into the metal than you would think. Simply sanding it off will not stop it.
I refurbished my boom over a year ago. What I ended up doing was grinding out the corrosion with a Dremel tool and having a welder fill in all the divots. After doing that I decided it would have been cheaper, and just as good a fix, for me to fill in the divots with Interlux Watertite Epoxy before painting the whole boom with Interlux Perfection.
Overall my fix has worked, except for three areas where the internal bails come out of the underside of the boom. Corrosion has begun to occur again under the paint surrounding the holes where the bails protrude. What I should have done while I had the boom stripped to bare metal was to completely cut out the area where the bails come through, including the holes for the rivets/bolts that attach the bails inside the boom, and welded in new aluminum plate shaped to fit the curve profile of the boom.
When it was all done I think I had as much in time and materials into the project as it would have cost to have a new boom made. I would check with Ballenger Spars and get a quote on a new boom, then you can decide if it is worth trying to fix yourself or if you can live with it the way it is for a while longer...
http://www.ballengerspars.com/