Concrete sealers penetrate up to an inch into the porous surface, forming an impervious barrier. Sealing not only helps prevent water damage, but it also helps keep stains from soaking into the concrete. Before sealing, the surface must be completely clean and totally dry. For cleaning oil stains, use a solution of a tri-sodium phosphate available from any home center. Or try scrubbing with fresh kitty litter.
Sealers come in a variety of sheens and looks. Test a small, inconspicuous area before sealing the whole slab. Apply the sealer following the manufacturer's instructions with a paint roller or a garden sprayer. The concrete will need to be resealed every few years if it's exposed to weather. Different sealers have different application methods and precautions. Be sure to read the label carefully.
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