Fasteners also prevent gases and liquids from leaking. When large areas or mating surfaces must be sealed, a gasket or caulking compound is used. In this case, the fastener provides clamping force but contributes little to the sealing action. If the fastener hole must be sealed, or the fastener acts as a plug, a separate element or compound may be added to the fastener.

Fasteners with preassembled, or built-in, seals include screws, rivets, nuts, and washers. Rubber, elastomers, plastic, soft metals, and interference fits are all used to maintain a seal. Sealing element plays an important role in cost. Designers should be particularly aware of the amount of time the fastener will have to perform the sealing function. Often, a lower-cost sealing fastener is satisfactory if exposure to gas and liquid is not continuous.

Choosing a material depends largely on its ability to resist attack from the liquid or gas it is to seal. Common sealing materials are: Buna N, neoprenes, butyl rubber, silicones, natural rubber, fluoroethylene polymers, polyethylene, and nylons. The characteristics of these materials should be carefully matched to the design requirements.

Temperature can have beneficial and detrimental effects on the initial seal. In low-pressure applications, moderate temperatures sometimes improve the initial seal. Prolonged exposure to higher-than-ambient temperature causes many seal materials to harden. Abnormally high temperatures can cause a complete breakdown in the seal.

Pressure causes mastic sealers or soft elastomer sealing compounds to flow. They are not recommended for pressures above 100 psi. But, when properly supported, the soft material resists 2,000 psi. "Soft" sealing fasteners are not recommended for applications where surface quality varies. Rubber and plastics can resist leakage in the full range of pressure from vacuum to more than 5,000 psi.

Corrosion is resisted by many formulations of rubbers and plastics. In several applications, stainless-steel fasteners are used with silicone or rubber, or fluorocarbons.