Before you can paint your model, you have to prime it. This enables your paints to stick to the plastic properly. Skipping this step can result in paint beading up on your model or rubbing off easily once dry. The easiest way to prime your model is with a spray can of automotive primer. I prefer Rustoleum grey primer, but there are plenty of options. Before priming your parts, you will need to mask off any areas that will be glued. Masking tape works great, as does painter’s tape or silly putty. Take you parts to a well ventilated outdoor area for priming. Apply several light coats of primer from different angels, allowing to dry between each coat. Make sure to cover all surfaces. If you don’t wish to use a spray can, primers are available for air brush or paint brush.
Once you prime your model, you will most likely notice some mold lines or gaps you missed. They really stand out at this stage. Go ahead and putty and sand any imperfections. Once you are happy prime that spot again.