(Continuing to: Sealing Natural Stone From The Marble Institute of America)
Exterior stones - Generally, topical sealers are not recommended in exterior applications, because they can trap moisture within the stone which may lead to surface deterioration during freeze/thaw cycles.
Resined stones - In some situations, impregnating sealers applied to a resined stone will cause the material to cloud, discolor, or fade. While this is a new topic in the stone industry, some initial reports have indicated that the application of a color enhancer may hide the problem. Additionally, the problems have occurred with solvent-based vs. water-based impregnators. It is highly recommended that you consult with the Sealant Manufacturer prior to applying a sealer to a resined stone.
source: Marble Institute of America