Sealing flagstone outdoors is
not recommended. Topical sealers
applied to stone that has already been installed may actually enhance the
stone's susceptibility to staining and discoloration. Sealing stone
outdoors leaves no escape route for moisture vapor that may enter the
stone from the bottom. As a result, moisture is trapped and may cause a
white film to develop and even cause parts of the sealer to bubble or
peel. Once peeling occurs, removing the remainder of the sealer that does
stay contacted is a very laborious task and requires additional chemicals.