I use a small amount of acid in a shot glass to minimize the vapors from a larger container getting into the surrounding area, primarily due to the fact that it'll rust stuff almost right before your eyes, and ya don't really wanna be breathing it.
Don't re-use/re-dip the same q-tip after it's been swabbed around on the aluminum because it will contaminate your container of clean acid and make it much less effective for additional applications.
Also, water with baking soda is the best neutralizer if you get acid where you don't want it.
If it's a heavy duty job and there's aluminum transfer all around the ports or transfers, smear petroleum jelly into those areas to keep the acid outta there.
And don't use acid where you may suspect (or see) pitting in the cylinder. It'll just make it worse and probably kill any chance of restoring the cylinder.
Do this in a well ventilated area. The fumes from the container are bad enuff, but the fumes from the acid reacting with the aluminum are worse.
When your're finished, wipe it down with a Scotch-Brite pad real well and wash it in soapy water with a bottle brush or sponge and rinse it like crazy.
Good luck.