Exterior Need Priming As Much If Not More Teen Interiors. The Sun, Rain, Heat, Cold Different Weather Conditions Will Take A Toll On Your Expensive Paint Unless You Prime The Surface Underneath Well. Use Magitech Build-up Exterior Wall Primer To Prepare The Rough Surfaces Outside Your Home Or Office Building Into A Smooth Canvas Exterior Painting Is A Laborious And Expensive Exercise And Proper Primer Coats Will Ensure That Your Investment Does Not Suffer Peeling Or Cracking.