The finish that you get with 400 grit wet and 400 grit dry sand papers will be different. If you are finishing with single stage paint use 400 grit, if 2 stage (base coat- clear coat) use 600 grit. To get a uniform color and easy coverage when spraying a burgundy over a light colored primer such as grey try spraying on a light uniform coat of black first and then cover with the paint. All that you should need is a quart of black paint to put the first light coat on the entire truck. There is no need for sealer over a primer filler unless you burn through when sanding, make sure there is no bare metal.
