What is used where faying surfaces are being fastened together?

• A. A lubricant.
• B. A sealant or jointing compound may be called for.
• C. A structural adhesive only.
• D. A paint primer.

Answer: (B)

When two parts are joined, the surfaces that meet (faying surfaces) are often sealed to prevent moisture and contaminants from getting between them and to protect against corrosion. A sealant or jointing compound fills gaps and creates a barrier, making the joint watertight or airtight as needed and providing a smooth surface for finish coats. Lubricants are used to ease assembly and reduce galling, but they don’t seal the joint. Paint primer protects exposed surfaces but isn’t applied at the joint to seal faying surfaces. Structural adhesive may be used in some designs, but isn’t the default solution for every joint. Therefore, using a sealant or jointing compound is the appropriate choice.