No one uses fiberglass matt and resin anymore in autobody work(professionals). The quick fix home guy does for bridging holes and floor boards, but no one else.
For bodywork use Evercoat brand fillers. Evercoat has a wide range of fillers to hoose from. From lightweight plastic fillers, fiberglass filler, fiberglass strand(tiger hair) filler to an aluminum based filler called Metal 2 Metal. Go with the evercoat and depending on why your using it and where will make the choice of type of filler for you. Lightweight plastic filler is best for smoothing and last smooth coat and is easy sanding. Both fiberglass is best for building and strength, but very tiring to work with. Metal 2 Metal is about a happy medium, its strong lightweight, still harder to sand than plastic filler, also a bit tougher to mix, but a good strong smooth filler. I recomend them all