Before you should even start to think about actually painting your miniatures, some care in the 'preparation' should be used. A few things need to be looked at when cleaning up your models, especially if they are multi part kits.
Removal: I have come to realize how easy it is to damage plastic miniatures when removing them from their sprue from my early days of modeling. Since then I attack a sprue with a modeling knife in a particular fashion. Firstly I look at the miniature to be removed and find where I need to make the cuts. By this I mean what cuts should I make first. The solution is simple. The first cuts should be at places where important parts of the figure meet the sprue. Such places maybe the top of a head, a weapon, or something that I know I want to be a clean cut. The reason that you should do this is because the miniature will remain stable as possible while you cut these parts, but as you get to the last few, the miniature is almost free of the sprue and can become unstable. So by this stage the last cuts to be made should be the least important area of the miniature, such as the bottom of the tab (the bit that slides into the base). If I cut too much off by accident, it doesn't really matter, as this part of the miniature people won't see.
NOTE: The utmost care should be taken when removing miniatures from a sprue. A sharp hobby knife is a must, so that the plastic is actually cut, and not torn. The sprue should be placed on a level, flat surface, with some type of protection, such as a wad of newspaper. All cuts should be made in a downward motion, away from the body. Fingers should remain clear from the blade. Do not force the knife in one hard motion, but push gently. You will find that the plastic will cut like butter in this way, and the risk of breaking miniatures is eliminated.
Cleaning: I prefer to keep using the hobby knife for this job, rather than needle files, but either will do. The only thing that you will need to clean from plastic miniatures is the mold line. Mold lines are thin lines of plastic that are left behind when the miniatures are made. They are quite easily found. If your miniature has a mold line, you will see it running the outside perimeter. It will usually start from the outside of one foot, continue over the top of the figure, and down to the other foot. If the model is in a pose with it's legs apart, there maybe a mould line around the inside perimeter of this also.
So how do I clean plastic mold lines? Easy. By using my trusty modeling knife, I drag the blade along the line, lightly, so as to scratch it off. You will need a sharp knife for this also, so make sure you scrape away from your fingers and body. After several passes, depending on the thickness of the line, the area will now become flat, and smooth. Viola, your plastic miniature is now ready to put in it's base.
Here is what mold line can look like. This Skaven has a line running the length of it's head.
Cleaning: A little bit more attention needs to be paid to metal miniatures. They may still have mold lines, like plastics do, but to remove them I would suggest Needle Files. As they come in different shapes and sizes, there is no part of any miniature that they won't be able to clean up. The most important thing with files, is to be watching what you are filing. This may sound a bit silly, but I have seen modelers take off more than what they were supposed to.
As well as mold lines, metal miniatures may also have 'Flash' on them. Flash is the name given to small pieces of metal that have escaped the molding process (due to high pressure), and are barely clinging to the model. The most common place to find these are on the front and back of the feet and any other appendages or weapons that end at a small area, like a marine raising a sword, there could very well be flash on the tip of the sword. This Bolt Pistol shows that unwanted flash must be removed.
To remove flash all you need to do is trim it off with your hobby knife. Again, common sense should prevail. Place the miniature on a flat, level surface and cut in a downward motion, away from the body, keeping your fingers out of the way. You may feel uncomfortable cutting so close to the miniature, so just cut off most of it, and remove the rest with a needle file.
Multi-part Kits
Cleaning: Multi part kits form the excitement when it comes to war game modeling. GW bring out a huge range of MP Kits, and their contents greatly vary. They range from your typical trooper with multi poseable arms right up to the Imperial Basilisk and Eldar Grav Tank. The only common thing between them all is that they are either made from plastic, metal or some of both. The cleaning methods for plastic and metal miniatures above can be applied to what you will get in your kit. But the main emphasis is on mold lines. You will need to look at each individual piece to find where the mold line will most probably be. If they are left behind, they will not give the justice that your finished figure should deserve.
Assembly: Once your kit has been cleaned to your satisfaction, there are a few more steps that you should follow. The first one has to do with plastics. The first thing I do with my large plastic parts is wash then in luke warm water with a very small amount of detergent, and then rinse in clean water. This will wash away any chemical films that are left behind from the pressurized injection process, and let your paint retain a smooth finish when it is applied. If they are left unwashed, and are painted, blemishes can appear in the paint work, making it look slightly unattractive (mind you, any Chaos players may like to leave this effect?)
So now all your pieces are ready for assembly. But before gluing anything, I would recommend that you prepare yourself for a couple of steps. Two things need to be looked at. Gaps and Pinning. Firstly you should determine whether any of the pieces will need pinning for extra strength. Sometimes the glue alone isn't strong enough to take the weight of larger pieces. To do this you will need two things. A Pin Vice with small drill bits and some sturdy wire, like the stuff coat hangers are made of. If you want to pin two pieces of a model together you will need to drill a hole into each of them, making sure that they line up together, with the model's pieces in the right position. Then cut a piece of wire ever so slightly smaller in diameter than the holes (you want a snug but not impossible fit). It should be the length of the two holes combined. Take the wire and glue it into one of the holes. You should have about half the length of the wire sticking out. When this is dry, glue the rest of the pin into the other hole, making sure that you have glue on the surface of the pieces that will come together. Hold it together so both pieces are in the right position and wait for it to dry. Voila, one pinned model. It isn't as difficult as it may seem. The tricky part is getting everything to line up properly. Holes, pin and model.
Another thing that you will need to be aware of is that all the pieces fit snuggly together. Some MP kits have problems coming together neatly, and parts can have gaping holes where it should be shut tight. The large vehicle kits, like tanks and bikes, should fit together without to much of a problem, and your standard troopers shouldn't either. But the kits that I find cause the most problems are large metal ones, such as The Avatar, The GUCO and Orion. When assembling these particular type kits, first see if the pieces fit together, without leaving any gaps. If they do fit, no worries, but if they don't you might like to consider a brand of filler. Such things as air drying modeling clay and two part epoxy fillers are quite a common tool for this type of work. It's just a matter of gluing your pieces together, following the instructions of the filler and filling the gaps. You don't need to be a sculptor to do this, just patient. The gaps shouldn't be too big (hopefully), and they will just require some smoothing over when they are full.
So as you can see, gap filling and pinnig are an important task with some MP kits. But just be aware that you will need to consider both at the same time, while you are checking your kit over.
