The heat gun is the tool that turns sheet foam into armor, and the first armor build guide gives its handling rule in three words: the gun moves. Held still, the surface scorches in place; swept constantly, the foam softens evenly through its thickness. What happens in that window is the whole trick of the craft, the foam's structure relaxes under heat, takes the curve you press it into, and keeps the curve when it cools. The guide's phrase for the behavior is The plain one: it remembers.
Two boundaries frame the window. Ventilation and gloves run the whole time, softened foam gives off fumes and the gun gives off heat in the direction you point it. And the sequence rule keeps the build safe: never reheat an assembled piece with glue inside before reading the glue's temperature rating. Contact cement's cured film and a heat gun in the same afternoon can be a fine pair or a ruined seam, and the tube's fine print decides which.
The guide's fixes table carries the best news in the craft: foam that warped wrong, warped because it was overheated once, resets. Re-heat it and press it flat under a board; the memory resets. The same forgiving behavior covers almost every forming mistake made before glue and paint go on. After those, the rules change: paint wants flexible systems and the seam wants fabric backing, both in the guide, but the forming window itself re-opens as many times as the foam stays unglued.
Sweep the gun, hold the curve, let it remember. Hang the mask. — the workshop crew