Filler mixing paper ?

Cardboard.

You shouldn't use cardboard for mixing filler because you don't know where that cardboard has been or what it has absorbed. I use a product called Clean Sheets, which is a pallet with a tablet of non-porous paper sheets that I can tear off and throw away when I'm done. Most cardboard is porous and will absorb some of the solvent in the resin (styrene), but most of all, the styrene will let loose any chemical that was already in the cardboard, whether it was a box that held oil, grease, or worse, silicone, as is found in Armor All. I once saw someone mix filler on part of an Armor All box. When he went to sand it with an air board sander, it peeled right off. And that's not to mention that he and everyone else in the shop had fisheye problems for about three days. Lexan, Plexiglas, or an old car door glass works far better than cardboard.

taken from

It's called an onion board. It's like a notepad but it's glued on three sides. Proper filler spreaders make a big difference too.

:o)