Clean the outside of the trailer using a wire brush to remove any visible rust. The more rust you can remove the better.
Use steel wool to remove the remaining rust. It may not be possible to remove all the rust.
Use auto soap mixed with water to wash the trailer. Remove all the dust that was created during the rust removal.
Let the trailer completely dry. You can use a soft rag to remove a majority of the water.
Tape off the areas of the trailer that you do not want painted. Cover the areas with newspaper and secure the edges with painter's tape.
Cover the ground with a drop cloth to catch any over-spray.
Use a commercial rust sealer on any remaining rust areas. Spray the sealer onto the rusted areas and let dry.
Spray a thin coat of paint on the trailer. This coat may not provide complete coverage. Be sure to paint all the edges and corners of the trailer.
Let the paint dry completely. This can take two to four hours. Check the paint. It should not feel sticky or tacky to the touch.
Apply a second coat of paint to the trailer. Wait two to four hours to completely dry.
Remove the painter's tape and newspaper.