Preparation in all its forms when painting bodyshells is very important. It is a common misconception that paint covers bad prep. It doesnít. 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.
Cutting and Trimming Polycarbonate
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 donít 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 canít bend it back, like on wheel arches, then gently tear the plastic along the score line. (fig 1)
Cutting the bodyshell out and mounting
Always mount the body on the chassis before painting it. While itís clear you can see whatís 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 donít mark the arches at all. If theyíre all marked just move one step on to marking the mounting holes. If youíve got one of the other two types itís 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 theyíre not marked.
Cutting the Arches
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 donít 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 itís 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 havenít 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 donít 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.
Washing the Bodyshell
Washing the bodyshell is very important. Manufacturers put a release agent on the moulds that the bodies are formed on and if you donít wash that off your paint will adhere to that and not the body, and will easily come off. Warm to hot water with some washing up liquid, then rinse it thoroughly and dry it with some kitchen towel.
If youíre body is only going to be one colour youíre ready to apply that now,but remember lots of light coats, allowing drying time between each. Laying one thick coat on will cause the paint to flake off after crashes. If you are going to use more than one colour a couple of points to remember. One, if you can always mask the body in such a way as to paint the dark colours first, and two, if you do have to paint a darker or similar colour on top of a light colour (say orange over light blue) always apply a blocking coat of silver over the first colour to prevent the second changing the first where it oversparays.
Masking for more than one colour
When masking panels on bodies it is a good idea to laydown an outline first using tape that has been cut down to 3 or 4 mm wide and then filling in with 18 or 24mm wide after. This method has two advantages. Firstly when tape is cut that narrow you can tease it into gentle curves, and secondly you can make sure that it lays down to the contour of the body, around body lines without coming away.
Mask the edge of the panel (pink) then fill in the area to be masked with tape. On this body the red was painted first and then the white.
Computer cut windowmasks
Most bodyshells now come with a set of pre-cut vinyl window masks. Some fit better than others, so you may still have a little trimming to do. If you find them tricky to get on right try this. Lift one piece of masking from the backing sheet and to check how well it fits lay it sticky side up on the appropriate window on the outside of the body. This way you can slide it around to find the best fitting position and even put some alignment marks down to help get it on right when you apply it on the inside.
Use the right tape
Donít use ordinary automotive masking tape when painting R/C bodies, because the adhesive layer is quite thick on this stuff and the tape itself is not completely flat. You will probably get the paint ďbleedingĒ under the masking, leaving somewhat less than a clean, sharp edge. There are plenty of paper tapes now available which do a good job so use them if you can.