One of the most asked questions I get is what is the best way to paint parts, so will attempt in this page to go through the different stages starting with preparing the parts and working through to finishing with painting them.
Now when casting parts, some are cast closed, where the mould has a fill hole and air escape hole, so when cured you have two extra bits of resin to remove, and some are open cast, as in part of the finished cast that is not seen is left open and the resin poured in here. Some casts come pre cleaned, while other have the casting and air spouts still attached, so these need removing first. I find a hacksaw is best to remove the bulk of it, then take down with a file and finish with some sandpaper. To be honest any decent casting will have those fill and air holes at the back so they are not seen anyway, so its up to you how well you clean them up.
Next you need to go over the whole thing with fine sandpaper taking off any sharp edges, as really shape edges, don't tend to paint well, as the paint tends when wet to leave that very fine edge. When sanding I only mean a smidge or so, just to take the point off and rough the surface and then the paint sticks fine.
Now comes the checking for air bubbles. Now pending the angle some things are cast at you may get tiny air bubbles in those awkward undercut places, or in bigger castings where the air in the resin rises once poured and gathers in flat or undercuts. I use car body filler, as it's a two part mix which sets in a few minutes and is easy to sand. I make up a tiny amount and using a piece of styrene fill any holes, then once set, sand smooth with fine sandpaper. I then check again to make sure I got them all, then using feel go over the part to make sure it's nice and smooth. I must confess to being a bit anal at this, as I used to work on car bodies, where you tend to use touch more than sight to check the body is smooth and flat, and some habits never go away.
Next comes painting and while you can use auto primer straight from the can you will get much better results by using a cheap airbrush. I have a cheap Revell one which works so much better than an aerosol can, as cans always seem to have a far too wide spray pattern, which is too heavy in the middle and too light around the edge. I still use the cans of paint, only I wrap them in paper towel, then puncture them with a square Bradawl, and slowly let the air out and when depressurised, leave overnight to reach room temperature and in the morning I have a can of ready mixed grey primer which can be poured into the airbrush pot and sprayed much more controllable onto the item.
Next thing is to clean the item being sprayed in warm water, and I find using hand soap the best as washes off very easily. However remember once washed not to touch it again with your fingers, always use either a cloth to hold it or a paper towel, as finger oil can leave marks on the surface. Now as for spraying, you need several light coats, as in dustings. I get many people say they spray things, and the paint separates and leaves little circles showing the plastic underneath, and this is normally due to them trying to cover in one thick heavy coat, and while it may seem to save you some time, it won't leave you with such good results. A heavy coat, will also be thicker in the middle of the flat areas than on the edges, where more often than not the underneath will still show through. Luckily I have a turntable I can place stuff on and turn round as I spray which makes things easier.
The first coat should be just a very light dusting, barely covering the surface, then the following two coats should also be the same, but between the three very light coats you will see its covering. Next leave it to dry for a few hours and then using wire wool gently rub it down, as this will flatten the surface without scratching it, then give a good blow, preferable without spitting on it, then give it another coat of paint. This can be heavier, so you see the surface shine, and if you position yourself just right you can see as the paint goes on it wetting the surface, which is a good sign. There is not set number of coats to do, spray till it all looks a nice and even grey, then leave overnight to fully dry.
Next again rub with wire wool to flatten the surface and remember not to touch it with your fingers, then get ready for the top coat. If there are deep or recessed areas, spray those and only those first. It's better to give those areas a couple of coats before you get on the easy to spray flats, as when you come to do the flat areas you can concentrate on just doing those, oppose to checking you got the recessed areas. The final coat should make the whole surface shiny and wet looking, then leave for a couple of days to dry properly. While most spray paint dries in a few hours it is really a couple of days before its properly hard, so spray your parts well in advanced, then when you come to fit them, you don't have to worry about marking the surface.