Eric, that finish is called "engine turning". It was believed that it relieved the stresses found in the skins of these planes caused when the skins were hammered into shape.

Three techniques are availalbe. One paint the cowling with one shade of aluminum colored paint and leave it, don't bother with the swirling.

Two, paint the cowling with a darkened shade of aluminum paint, let that dry thoroughly. Then paint over that with a lighter shade of aluminum, once that drys take a rubber eraser or a large buffing wheel on end, in a dremel tool and GENTLY swirl the two paints together.


Three, cover the cowling with aluminum tape from the auto hardware store, then rub the aluminum tape with the eraser in the dremel and replicate the swirls in the aluminum tape.

HTH

Tom