the only problem with using fibreglass resin is that it is not waterproof it is actually the gel coat that provides the waterproof barrier.The resin is used to give strength combined with the matt
yes there the ones there are a few products out there mainly used in wet areas like shower walls and walls around the bath saves on spreading with sand/cement. you can use normal plasterboard if your not using them in wet areas on the other walls and tile straight on top after a coat of pva
just a thought if the walls are that bad and need about 20mm of plastering how about bedding either a cement based backer-board if being used in a wet area or plasterboard if not in a wet area and tiling over the top of the boards.........be a lot quicker