Waterproofing a metal shed roof or any metal roof for that matter should not be done the moment the roof is installed. During the manufacturing process the metal roof has multiple applications of oil on the surface and this oil should be allowed to naturally wash off. You can wait for the next rain shower and sunny day for the roof to clean itself before applying a sealant for waterproofing. By following a basic process you can waterproof a metal shed roof in a weekend.
Set the extension ladder to the lower side of the roof in a comfortable position so you can safely climb on to the roof on a dry and sunny day. The warmer the day, the better the caulk and paint will adhere and seal the roof. Use the broom and remove all debris from the surface such as limbs, leaves and any dirt. The roof should be clean before applying any sealant or silicone caulk.
Place the silicone caulk into the caulk gun and seal up any holes or open seams in the metal roofing. You may want to "dab" some silicone on the fasteners, nail or screw heads, to create an extra seal beneath the roofing paint. Allow the caulk to fully cure, approximately 24 hours, before applying the paint to the roof surface.
Open the canister of metal roof paint and thoroughly mix as per the manufacturer's instructions. The metal roof paint will contain particles held in suspension. These particles are what bind together with the metal surface to create a good seal for waterproofing.
Attach the paint roller to the extension handle and carry it along with the paint to the upper edge of the metal roof. You will want to begin painting the roof on the upper side of the incline and work downwards to the lower edge where the extension ladder is placed.
Apply the paint in thick even strokes, blending the paint together as you proceed across and down the slope of the roof. You will want to work with some diligence in speed as the paint will quickly dry against the metal surface on a warm sunny day.
Check with the manufacturers instructions concerning cure time and applications of a second coat if needed. Throw the roller cover away after the paint is applied. Most roof sealants will be very difficult to wash out of the roller nap.
Follow all local regulations when disposing of any paint material.