WD 40 is the trademark name of a penetrating oil and water-displacing spray.

The long-term active ingredient is a non-volatile, viscous oil which remains on the surface, providing lubrication and protection from moisture.[4] This is diluted with a volatile hydrocarbon to give a low viscosity fluid which can be sprayed and thus penetrate crevices. The volatile hydrocarbon then evaporates, leaving the oil behind. A propellant (originally a low-molecular weight hydrocarbon, now carbon dioxide) provides gas pressure in the can to force the liquid through the spray nozzle, then evaporates away.

These properties make the product useful in both home and commercial fields; lubricating and loosening joints and hinges, removing dirt and residue, and extricating stuck screws and bolts are common usages. The product also may be useful in displacing moisture, as this is its original purpose and design intent.