What air pressure, and what torch do you have? Aluminum cuts with a plasma are not pretty typically and does leave slag under the cut because it melts, and does not "oxidize" the same way steel does. Are you holding a standoff? How much? It should be less than 1/8". Are you doing a lot of straight down piercing? Are you using the torch in the negative? Set for DC? What is your cfm at 90 psi?