Another way to produce Smooth ID ducting is to use child's modeling clay. Form up what you want in clay.
Smooth the surface with the clay warm to a perfect finish. Place in fridge until stiff. Paint on a thick coat of Styrene with baby powder mixed together and let harden. Cover with 4 layers of glass cloth and resin. Epoxy is fine as well and much stronger and more expensive. Same system as above.
Make a simple shape on a flat board and try it first. You can produce very complex shapes this way and remove the clay after complete with solvent. Good for custom air horns and ducts.
Lynn E. Hanover