Please note: I wrote this article some time ago for another site that I used to own. It's not really up to TG standards as it has no images to illustrate it. This is an issue that I hope to rectify as and when I have time to take suitable photographs but in the meantime I offer it here in it's original text only format:
The first thing to do is to wash the parts in warm soapy water using an old toothbrush to get into all the crevices. When resin is cast, release agents are used and these will affect the adhesion of fillers, glues and paints so start by cleaning them off.
Flash can be removed with a sharp hobby knife, file or saw depending on the quantity to be removed. Bear in mind that a saw or file WILL leave tooth marks in the relatively soft (as compared to the metal of the blade), resin and you'll need to sand these out. This is not at all difficult to do provided you've not cut or filed away too much material in the first place.
Sanding is done with progressively finer papers until you finish with 400 or 600 grit paper however you will probably want to do some filling too.
At this stage the filling is best done with epoxy putty. Most resin kits will contain a couple of air bubbles that need filling and sanding and a poorer quality one may have dozens. Don't be afraid to make the holes larger if this makes it easy to get a descent amount of filler into them.
Another thing to check at this stage is that all the parts are the correct shape. Resin parts are cast in flexible rubber moulds and while this has the advantage that if allows the casting of parts with considerable undercuts, the down side is that the mould can 'give' under the weight of the resin. By checking for this you allow yourself the opportunity of sanding down any surfaces that have bulged, filling any that are hollowed and even cutting, repositioning, filling and sanding to correct shapes.
Resin parts can be joined with super glue although a 2 part epoxy will probably be better because chances are that the parts won't match up particularly well. Start by dry fitting the parts together to see what's needed.
Because of the weight of resin, something else to consider is whether it would be worth drilling the joints and using metal rod to help strengthen them. Now, in case that sounds like a lot of trouble can I suggest that you've just had the same silly idea that I had when this was first suggested to me? The though that went through my mind was that trying to drill the holes so they matched up would be a nightmare. What I was doing was anticipating that if for example I was to use a 4mm diameter rod I'd drill 4mm dia holes. Doh! The trick is to drill something like 7mm holes for the 4mm rod. The rod isn't there to support the job WHILE it's drying. You can give it that temporary support with other things. The rod is there for AFTER it's dried so what you do is drill the holes and then fill them with epoxy just before pushing the rod in and assembling the joint. When the epoxy dries the rod is locked in place and strengthens the joint.
After the joint is made and the glue has hardened scrape away any glue that has oozed out before filling any gaps. At this stage, super glue can be used to fill small gaps with epoxy for any larger ones. A final round of sanding should prepare you for painting.
Painting resin parts is a fairly standard procedure. Another wash is probably in order and you can use enamels or acrylics directly on the resin, or preferably, because of the usual benefits, on top of a coat of primer.