Spray-up molding is used to make parts with large dimensions and complex shapes such as bath tubs and boat hulls.

The process consists of simultaneously spraying onto the mold, chopped glass strands, normally of 30 mm length, and the resin necessary for their impregnation. After spraying onto the mold, rolling by hand is necessary to flatten or compact the glass-resin mixture well onto the mold surface and, as far as possible, eliminate air bubbles generated by the spraying.

The spray machine generally consists of:
- a glass chopper driven by compressed air
- a pneumatic pump for feeding resin to the spray gun
- a solvent delivery system for cleaning the gun