I’m not using the standard Velocity cooling plenum.  I decided to use a more traditional approach where aluminum baffles create a box that mates with the upper cowling to create the high-pressure plenum.  The interface between the aluminum baffles and the inside of the cowling is a flexible baffle material that creates a relatively airtight seal.  On most other planes the cowling is sheet aluminum and the flexible baffle will slide up against the cowling.

But my cowling is a rather course fiberglass. So to allow the flexible baffle to slide, I decided to create a smooth surface on the inside of the cowling… but only where the baffle will contact.

I sanded down the inside of the cowling where the baffle material will contact the cowling. Then I used masking take to define the area and painted on epoxy.

Once it dried, I removed the tape over the inner area, sanded the epoxy and painted on a second coat.

Now (hopefully) the flexible baffle material will slide as needed.