I want to use a mould 3D printed in HIPS high impact polysyrene. The surface of the mould will be sanded. Which of the release agents is most compatible with HIPS ? 

All our printed parts are rubbed down then sealed with marbokote 2002

apply five fairly wet coats (getting drier as they build) allow each to air dry naturally and allow at least half hr between coats and try to avoid runs.

when complete oven cure at 50/60 for 1hr

De nib and lightly abrade with 12/1500.

apply your last costly lightly and as dry as you can and allow to air dry overnight an you should have a Finnish like glass that you can then polish for an even better shine or use as is.

Then its just apply your release of choice as you would a "normal" mould

I use 3M,other people swear by other polishes but I'm happy to pay the premium and use the fast cut an the fast cut plus.

once you master the 2002 its unlikely you will need to polish but in my application its for turbo pipes for a f1 engine and the spec/Finnish is very high.

we are using freecote 700,but only because its what the company uses I would much prefer mono coat !

in our application the part is made in pre preg and bag cooked.

once cured we de bag and put the part and mould in a oven at 140 and at this temp the printed part turns into a marshmallow consistency and is pulled from/out of the part.

item looks perfect as if it had come from a carbon mould