Preparation in all its forms when painting bodyshells is very important. It is a common misconception that paint covers bad prep. It doesn’t. If anything it accentuates mistakes, so by taking time preparing a body the work will be easier and the finished job better. Here are some tips on the different aspects of preparation when painting bodyshells.
  
 
There are two methods of cutting the polycarbonate plastic that bodyshells are made of, either cutting with scissors or with a knife. Scissors are easy and now curved bladed pairs are available it is quite a simple job to cut around the whole body with a combination of straight and curved blades. Cutting with a modelling knife is not quite as easy but does give a better finish, and remember you don’t have to cut all the way through the plastic, just score it with the knife firmly and bend the panel back on itself and the plastic will snap apart. If you can’t bend it back, like on wheel arches, then gently tear the plastic along the score line. (fig 1)
 
 
Always mount the body on the chassis before painting it. While it’s clear you can see what’s going on with mounting posts and get an accurate mark on where to make the mounting holes.
Start cutting out by trimming just the marked base line all the way around the body. Wheel arches vary from body to body, some manufacturers mark them all, some just mark the front arch and some don’t mark the arches at all. If they’re all marked just move one step on to marking the mounting holes. If you’ve got one of the other two types it’s best to take the body posts off the chassis, lay the body over the chassis (you may want to block it up to get the clearance right) so you can either a) line up the front marked arch with the front wheel and then mark out the rear, or b) line up the body to best fit front and rear aches if they’re not marked.
  
 
Non marked arches are best cut out using a compass that is fitted with a blade instead of a lead (bigger stationers sell this type of compass), so when it comes to marking out just put a dot over the dead centre of the wheel nut and just cut the arch slightly larger than the diameter of the tyre.
  
 
Once the arches are marked out put the body mounts back on to the chassis but don’t remove the waste material yet. If the body posts have been used before they will be cut to the right height but if they are new it’s best, although a little fiddly, to trim them down to a little bit longer than the finished length. Now rest the body on the posts making sure it is at the right attitude by ensuring the bottom of the shell is parallel to the work surface. If you haven’t got the post lengths dead right the body may rest just on the rear or front pair. Use the centre lines you marked for cutting the wheel arches(that's why you don’t pull them off yet!) to be sure the body is correctly aligned to the front and rear wheels. Make sure also that the body is central side to side by making sure the gap between the outside of the wheel and the inside of the body is the same on either side. Now looking through the bodyshell mark the positions of the body mount posts.
You can use ordinary twist drills to make the holes in the body but they have a habit of skidding across the body before they start cutting and scratching it. There are many types of tapered body reamers available now, and if you compare the cost of one of them to the cost of a body it makes good sense to have one to save the risk of marking the shell whilst trying to make the mounting holes.
Last jobs are to cut the wing out (if the body has one) and drill the screw holes to mount it.