If it is a quality bedcoat like Rhino-lining or Speed-liner the best way to remove it would be a power wire brush. This would be the safest for the panel and the person doing it. If it is one of the low quality coatings for the DIYer you may be able to remove it with paint stripper. I would try a 3" electric grinder with a cupped wire brush, this is how we remove it to do bed repair with a bed coating. Remember to use safety glasses and a dust mask or respirator.