This may sound stupid,but I use drywall compound mixed with a little furnace cement. I thin it with water till it's like a melting milk shake.You only want to coat the part about 1/16 thick or so. If your part has little nooks and crannies you may want to paint it on with a brush.
It turns out just as smooth as the finish of the foam.The better the finish is on the foam the better the part will look. Foam is easier to work than aluminum so take your time on the foam plug and your finished part will require little to no machining.