The term 'make airtight' can be accomplished by using the crossword solutions CAULK (5 letters), SEAL (4 letters), and sealup (6 letters). Caulking is a process of filling in gaps or cracks with a material such as silicone, which is used to prevent air or water from passing through. Sealing is the process of applying a material such as glue or wax to a surface to prevent air or water from passing through. Sealup is a combination of sealing and caulking, where both processes are used together to make a surface airtight.