if you're doing aluminium the key is cleanliness cleanliness cleanliness. did I say cleanliness?

get some acetone and stainless brushes (must be stainless!). the little packs of 3 small brushes you get from halfords / ebay or the local Pound shop typically have a stainless brush which is ideal. use it for nothing else but scraping off the oxide layer on the aluminum AFTER you have cleaned the metal first with the acetone. if you don't do that it may end in a puddle of the garage floor!