Pre-Sealing
If the porcelain tiles lack any kind of texture and have a smooth surface, use a pre-sealant. You paint the pre-sealant on each tile and let the product dry before installing the tiles outside. Textured tiles do not require a pre-sealant as the sealant may actually damage the tiles. Check each tile carefully for any clear coating or sealant placed on the porcelain by the manufacturer. This coating keeps grout from sticking to the tiles during installation. Tiles with this protection do not need a pre-sealant as the sealer permanently adheres the manufacturer's coating to each tile.
After Installation
Whether you pre-seal the tiles before installation or install the tiles as-is, you must apply a sealant after installation. Wait until the grout dries to prevent smearing the grout onto the tiles. Apply the sealer to the tiles with a paintbrush or pour the solution into a paint sprayer. Most products do not require any rinsing; but after the sealant dries, gently buff off any debris with a soft cloth. The soft cloth will not scratch the tile.
What To Use
Several companies make products suitable for sealing outdoor porcelain tiles, including Custom Building Products, TileLab, Trewax and Miracle Sealants. Only use products designed specifically for outdoor spaces, such as indoor/outdoor stone and tile sealer. If the product is only suitable for tiled surfaces, use a separate grout sealant. Read the manufacturer's guidelines carefully and make sure that the product works on porcelain tiles. The products keep dirt and debris from attaching to the tiles and makes cleaning faster. Rinse the tiles with water; no scrubbing is required. The sealant also keeps snow and ice from damaging the tiles.
Considerations
Sealant gives porcelain a slightly glossy surface as it dries, which changes the overall look of the tiles. A darker-colored tile might look lighter with the addition of a sealer. When making your tile choice, keep this change in mind. Opt for a color that is slightly darker than you originally wanted to compensate for the color difference.