Fiberglass Vs Fiberglass reinforced body filler

I am plannning on fiberglassing some areas in my new sheetmetal bed (areas where i can't contor the metal the way i like. ) My friend is telling me that we should use some fibergalss to reinforce everything, and it will look smooth as hell. But does fiberglass sheet/resin and sheetmetal mix? I know that the two act differently in temperatures, so will i have a problem? I was also thinkin of using this fiberglass reinforced body filler i have (fibral). it is meant for bodywork, but will it be as strong/better than straight fiberglass? t