WHAT IS A BODY SHELL
This is the cover for your rc model which has 2 basic functions, the first one to personalise you model and secondly to protect all the electrics / mechanicals within the model.
These are generally made from either abs plastic or lexan, I’ve not had much experience of abs plastic shells but these are a moulded opaque shell, these are painted from the outside so the paint can get scratched in collisions and crashes where as lexan (polycarbonate) is a clear plastic that is painted from the inside.
Most shells are designed to suit a particular scale or car design ie on road or off road, they are usually specific to a certain make and model to make fitting easier.
Preparing to paint them is the same basics, wash with soapy water to remove any residue oils, then you can either send it to you local rc painter or have ago yourself, if your doing it your self please ensure you use the correct paint for the type of shell as the 2 types of materials require different types of paint, you can use a good quality masking tape for the simple designs or use a liquid mask for more complicated designs.
Rattle cans or aerosol paint needs to be applied in very light coats , ensure the edges of the masking are firmly adhered to the shell or the paint will bleed under the masking.
Water base paints tend to be used by the painters and applied using an air brush and compressor.
There is plenty of info on youtube on the art of painting shells.