Whoa there! If I read you right youv'e got it wrong!
Take a second and reread what I was saying...
If you have a mold on hand and it is made of Epoxy resin. just wax it with mold release (I use Grignard' s Mold Magic) and layup your part from epoxy, do not use 5 minute epoxy and do not use polystyrene)! 60 min at the minimum; West Systems or the like is what a lot of folks use.
If your mold is made from poly you can use either epoxy or poly for the part.If the part is a servo cover you can use 2oz. cloth to make the part adding additional layers to get the thickness and strength you want or go to a heavier weight of cloth.
Do not sand the inside of the mold with anything heavier that 320 WD and then progress to 3200 grit.
If you have an existing part that you want to strengthen, then sand the inside if the part, not the outside and add additional layers of epoxy and cloth, again use 60 min or better epoxy.

You are welcome to PM me if you would like.

Brian