I have an extremely old metal roof on an old historic home that has had various coatings and patches done over the years; looks to be Kool Seal aluminum type coatings and rubberized/elastomeric wet patch type cements for the most part but hard to know for sure. Would it be safe to use your product, in conjunction with the primer, to go over the previous work, or would all previous coatings/repairs have to be taken down to original metal prior to application?