Urethane foam, the blue/pink/green stuff produces toxic fumes make sure you have good ventilation. It is also stronger and sands well - wear a mask!

If you are looking for cheap and cheerful try covering with brown paper and PVA. Strong and easy to repair. Excellent for models up to about 25 and some larger deltas.

I like to cut with color markings on the templates to be sure that I am getting an even cut. I sit down with the foam lightly clamped down and against an end stop. I pull the cutter towards myself. with a small amount of downward pressure. Do not pull hard, let the hot wire cut but do not stop either that gives a ripple.

These are all built with blue urethane foam and brown paper. The Lanc has 4 x 15s and is a veteran of many airshows. It was built By Barry Beech of Mainly Models.