I'd like to get the back part of my mould painted, but I've no idea how to get a smooth finish on that side of it (the peel ply side) Do I sand it down? Brush another layer of resin?

are you talking about the backpart of your mould or from the part?

Makes it a bit more difficult...

Sand down and a couple of heavy coats of lacquer would be how I go for a one off

There multiple ways:

1) sand and lacquer.  This is how I'd do it for a one off

2) remould using a vac bag rather than infuse and be very careful laying in non-perforated release film (should give a smooth finish)

3) Make two mouldings and bond them together (could be tricky)

4) as suggested above mould each side and use a closed mould system

Hi!
I think good way to do it is just to skin/wrap back layer with carbon - as they do in CarbonMods. Put resin, wait until it gets tacky, put carbonfiber cloth, another thick amount of resin and use non perforated film or vaccum bag as they do in this moment http://www.youtube.com/watch?v=Xm2Ezo70vFI&feature=player_detailpage#t=405.

~~~Everytime I hear sound of demoulding part I have eargasm~~~

if you want to smooth layer of backside,

Sand and lacquer or sand and thin coat of resin are the two simplest methods.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant