not being mean but sounds like you are either adding to much hardner or not working it enough before you apply it to the case
first thing to make sure is to add about a pea size piece of hardner to a golfball of filler
and work it to amke sure it is properlymixed together but here comes the difficult bit if you work ti to much you could have the opposit affect as it has had time for pieces to dry in it.
also i have hear fibre glass works well someone on here not long ago made a whole case out of it might be a idea to ask him sorry i couldnt be more help