Smooth around the edges

The airfoil templates will guide the hot wire through the foam core section. The thin wire tends to catch on the fiberous edges of the plywood and if the wire pauses against the ply for even a moment, it will burn a groove in the wood.

You can make templates out of phenolic sheet or other dense and heat resistant materials, but we don't expect to use these particular templates more than once so we went the cheap route.

I sanded the edges with 600grit sandpaper and then trimmed them out with aluminum tape from the HVAC supply. The wire slides much better and a pause won't burn a groove nearly as fast. It's not perfect, but it's darn affordable.