Although Duncan Ceramics is under new ownership its products are still available online. They have multiple sealer products and many years of experience.
Many ceramics are either porous by nature or by necessity. For example, stonewares need to be non-vitreous enough that they do not warp or blister on firing. Red earthenwares must be porous in order to have the red color (they go brown when fired higher). White talc or dolomite low-fire clay bodies always have high porosity. Bricks must have minimal firing shrinkage, which guarantees substantial porosity. Even porcelains can blister and it is common to cut back on feldspar to give them more margin for overfiring - that brings porosity. If water penetration must be prevented all of these need to be sealed, these are some of the methods.