I don't think fillers necessarily have to be from the same company that made the rest of the stuff. It seems a lot of guys like Rage, and I have had good luck with Evercoat Base Coat/ Clear Coat filler. But it is a good idea to stick with one line for your primers, catalysts, reducers, paint, etc. That way you not only prevent compatibility problems, but you also don't give the paint manufacturer a reason to be able to blame any problems on you not using their matched components.

Don