Here is the process and supplies you SHOULD use, but not mandatory.
1. Trim and fit the body (mounting hole locations done here too).
2. Sand and smooth the edges, all the way around.
3. Wash the body
4. Mask the entire body
5. If using liquid mask, draw your design on the outside of the body
6. Paint
7. Back your paint
8. Peel the protective film
Trimming
I find it helps to use an ultra-fine Sharpie and trace the cut lines before I start trimming. Sometimes the cut line can disappear in certain light, depending on the angle the body is to the light.
You can either use scissors, or an X•acto to do this. I typically use a combination. Scissors on the things I can get to, straight edges, etc, then use an X•acto on things I can't get to with scissors.
When using the X•acto, you only need to lightly score the line. When doing wheel arches, cut around the line with the knife, then you can use relief cuts to break the wheel arch out in pieces.
Notice how in the pic above the relief cuts try to break the curved arch into straight segments. You will have a harder time snapping the segment if there is much of a curve in the snap line. Don't tug and tear the pieces out. Bend back and forth gently and they will eventually break free. You can do this without the relief cuts, but I find it easier with them.
Once your body is trimmed, put it on your RC and line it up with your wheels. Make sure your steering is centered, and the battery is installed so your RC is at the ride height it will be. Then pop your body mounting holes in with a body reamer.
Put your body pins in and turn your steering and compress the suspension and check for any tire clearance issues. Trim where necessary.
Sanding and Smoothing the Edges
Go around the edges with wet/dry 220 grit sandpaper or enery cloth. Knock out any rough spots. Smooth your wheel arches and radius the corners where the arches transition to the lower valances (areas under bumpers) and rocker sill (area under the doors).
Once everything is smoother, go around your edges with some 400 grit wet/dry sandpaper. You're trying to create a bit if a radiused edge inside and outside the body.
Wash the Body
Get a green Scotchbrite sponge and some dish soap. Wash the body inside and out with the sponge side and some soap, being mindful of the edge of the protective film. Always wash the outside from the body towards the edge so you don't peel the film.
On the inside, after the preliminary wash with the sponge, you can flip it over and use the Scotchbrite to lightly scuff the inside. You should barely see the scuffing, if at all. Do the entire inside, making sure to get into the nooks and crannies.
Dry the body thoroughly with a clean lint-free towel and let it sit for a few hours, or you can hit it with a hair dryer. Be careful. A hair dryer can get hot enough to melt lexan. More air, less heat. Hair dryers can also create water spots if you have hard water
Masking the Body
Once the body is dry it's time to mask the body. You can do this two different ways. Liquid mask, or vinyl masking tape.
One Color Designs
If you're just painting the body one color, you can simply apply your window masks and spray the body.
Simple Paint Designs
If you're doing a fairly simple design using a couple colors, vinyl making tape and using your window masks is fine. Here is the vinyl masking tape I use.
https://a.co/d/7S5I2aM
Liquid Mask
If you're doing a more complex design, liquid mask is much better. For this, you don't really need to use your window masks.
Apply an even coat of liquid mask to the entire inside surfaces or your body. A set of horsehair or "camel hair" brushes is ideal for applying liquid mask. Note: Camel hair brushes aren't really camel hair. It's a combination of animal hair.
https://a.co/d/afuZRLp
Let that coat dry thoroughly. You want it to be clear before you apply your next coat. This can take hours, so be patient. Put your body under an air vent, or next to a fan. You want at least 5 to 6 coats total.
Once your liquid mask is applied, draw your design on the outside of your body shell. Make sure to also mark the window and trim edges, along with any body details like hood vents, grille edges, lights, etc, if you plan on leaving those areas clear, or plan to paint them a different color.
Once all your line work is done, use an X•acto with very light pressure and trim out your entire design. Be careful not to peel up any edges. If you do, you can apply more liquid mask there and let it dry, and recut that area.
Peel the section for the first color and spray. Typically, you want to start with your darker colors and work towards your lighter colors. But this isn't always the best method. If you have to paint a lighter color first, back it with silver before spraying a darker color over it.
Painting
Spray your first coats very light. You should barely see the paint. Make sure to get your mask edges sprayed. The trick is to lightly spray your masked edge, without getting the paint heavy enough to run under the edge. Doing this in two or three coats will seal that edge, so you don't get paint bleed.
References:
Prep - Mark has some great tips in this video. Pay attention to how he shows his method for lining up your window masks perfectly.
Paint and Decals - Again, Mark shows you some more decalling techniques. This guy is really good at applying very complex decals and getting them to lay exactly where he wants them to go.
Liquid Masking - This is a video I learned a few things from. There are other (and better) videos out there, but watch this one too.
