Completely remove old decals. Do not try to remove them with a sharp object. Heat them with a hair dryer or heat gun, and carefully pull the decal to lift it from the sidewall. Avoid overheating the area and risking damage to the sidewall. Do small areas at a time. Once the decal is gone, remove traces of adhesive with 3M Adhesive Cleaner, then wipe the area with denatured alcohol.
(this was from Trailer Life website).
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