We have found that for Steel Roofs, Inland Coatings work best as they have an elongation of up to 500% so that they can expand and contract with the movement of the steel. As well, we don't need any primers and we can apply this over rusted steel after carefully preparing the steel roof. For Flat Tar and Gravel Roofs, we have some other coatings we use to help extend the life of Flat Roofs but it is only a short-term solution.
In our experience coatings work best on Low Slope Metal Roofs such as Butler Roof. We have done several small residential Steel Roofs that are Low Slopes on trailers, sheds, cottages, homes etc with great success. As well, we have completed commercial roofing projects on such projects as Municipal buildings and RV dealerships that wanted to extend the life of their existing commercial roofing but didn't have the budget to retrofit them with a membrane.