I'm  making something similar to this


which holds this



So I need to take a mould off the back of the unit above to get the profile.  It needs to be a snug fit in the holder and I'm wondering what thickness of wax sheet would be best to get the profile correct, 2 mil or 5?

Also when the wax sheet is on, I've seen that some put packing tape over it to prevent sticking, is that a definite method because surely it means sanding work on the mould to remove any lines caused by the tape?

All depends on how thick you want the finished part. Use 2mm wax if you want the finished part to be 2mm etc etc.....But as long as the wax is warm it should conform to the shape no matter what thickness you use. But watch for the thicker wax splitting on sharp angles.

Yes, i sometimes use brown packing tape over the wax. Resin will not stick to it but you'll need to refinish the mould. You can also use plenty of PVA.....and im talking lots of it to create a barrier between the wax and resin.