I think I'd apply it over the filler. The filler would keep the cotton from wicking resin out of the Quick Fair and bonding the canvas to the stem (cotton canvas loves to suck up epoxy). The bond between the Quick Fair and filler should be adequate (at least as good as the bond between filler and more filler would be). We generally don't apply epoxy over other finishes because in that case, the epoxy bond to the surface is only as good as the bond of what it's applied over - but in this case, I think that would be pretty much what you're looking for.