absolute bolloxMultifinnish on sand cement. Board finish only on board. Damp. Down the s/c first if it's been on a while or just give it couple of coats of water down pva.
just because it says board finish on a bag does not means it's just for plasterboard
you can use either finish on anything, the only difference between multi and board finish is you get a bit more working time with multi...board finish sets a bit quicker on board as it's classed as a low suction background.
even BG's own guidelines state to use board finish on their Dri-coat when dried out which is guess what? cement based and dampen with water first