A horse trailer in need of painting is more than an eyesore; it can be dangerous to the animal if flakes of paint are in an area where they can be eaten. Repainting a horse trailer also greatly raises its resale value if you plan to sell it. The process is just as much preparation as it is actual painting: if you skimp on the first part, you'll pay for it in the second. Take your time and do the job right.
Scrub the entire trailer, inside and out, with a garden hose and scrub brush. You want to remove any dirt, animal waste and other contaminants that will get in the way of the new paint.
Sandblast the entire trailer, removing rust and old paint. Lightly score the surface so that it will accept paint more easily. You can sand by hand or with an electric sander if you want, but it will take a lot longer.
Spray the trailer one last time with the garden hose on high pressure to remove any dust left by the sanding process. Allow the trailer to dry fully before continuing.
Block off windows, tail lights, fenders and any other areas that you do not want painted with masking tape. Use a razor blade to trim the edges clean.
Add white automotive primer to an electric paint sprayer. A sprayer will allow you to work more quickly than painting by hand, with no danger of leaving brush strokes behind.
Apply one or two coats of primer to the entire interior and exterior of the trailer, depending on the coverage that the primer provides. It is always better to apply thinner layers than thick ones to avoid drips.
Allow the primer to dry completely. During this time, clean out the paint sprayer.
Add a light-coloured automotive paint to the sprayer. Brighter colours are more welcoming to horses, especially on the inside of the trailer. Horses instinctually avoid caves and other underground areas, so if you use dark paint inside the trailer you may have trouble getting your horse to go in.
Paint the trailer with several thin coats of paint. Allow each coat to dry before moving on to the next.
Allow all of the paint to dry.
Carefully remove the masking tape.
Always wear a respirator and goggles when using a paint sprayer. Let the trailer sit open for several days to ensure that all fresh paint fumes have dissipated before allowing a horse inside.