Grinding aluminium is a sure way to screw up the disk/wheel. Also, most grinding wheels and sanding disks are aluminium oxide, which is the very last thing you want around when welding aluminium.
Use silicon carbide wheels or disks or -- generally a better bet -- a burr in a die grinder.
I'm not sure if this is valid advice but I have a stainless steel wire brush that attaches to a drill. An agressive pass down the area to be welded to remove the oxide layer and cut contaminated area makes welding a doddle by comparison to any other method I have used. More than anything else having the surface roughenned up in this manner makes the weld pool so much easier to see as it comes up like a mirror against the roughenned surround.
alum has to be very clean to weld. it my be because the grit from the disk is inbedded in the alum. always the 3 rules of welding alum
1: clean
2: clean
3: clean