I've gotten good results on think material using a bigger endmill (.25"). If you're getting a big burr, you're not getting cutting, it's more like the mill is sort of plowing through the material and pushing it out of the way. Aluminum is pretty sensitive to lubrication, exact alloy, speed, feed, and cutting tool. I've been lubricating with kerosene recently, and it works well for me. Some people swear by alcohol. Others get just fine results with WD40.
G-Wizard says 32 inches/min, 17k RPM for a .25" carbide endmill, .06" depth of cut (so 1 pass on your .020), using an plain carbide end mill. Use something coated (no Al coatings), and it should work better. Do be careful that it doesn't rip the material right off the wasteboard, the forces are likely to be kind of high, and if it's not stuck down well, it might come off.
Is there a particular reason for using such a small end mill?