I have always heard not to mix body filler on anything like card board, or wood, as both will soak up some of the chemicals rendering the filler less effective.



IMO, I don't think the filler is there long enough {or wet enough} for the card board, or wood to absorb too much to render it less effective.



What say the forum?

 

I have mixed all of it on porous surfaces with no ill effects.

 

Ive never had a problem using cardboard.

 

Upol make a product called onion board which is a large pad of paper making clean up a breeze as you just tear it off and throw it out...