for your masking problem, I would suggest to use Parafilm as masking material, or else
Post it notes. I think any masking tape will pull up an acrylic coat over MM alluminum.
If your camo has some soft edges, you can also airbrush it freehand.
FInally if your motivation for the aluminum coat was to do paint chipping later, then I feel that a moderate paint chipping can be done as well without an aluminum undercoat, using a silver pencil or a small brush with for example Humbrol silver paint.
IMHO an aluminum undercoat is needed only for badly chipped paint jobs, like WWII japanese airplanes, otherwise you dont need it.
best,
Christian.