McMaster 94708A517 Galvanized Steel and Neoprene Bonded Sealing Washer, Pack of 50, for 1/2" Screw Size, 0.532" ID, 1" OD, 0.086" to 0.116" Thick

These washers create a stronger seal than metal or rubber alone to lock air and fluid in, while keeping dust and contaminants out. The flexible rubber layer compresses to fill gaps, while the metal backing stops the washer from squeezing through the hole. These washers are a great solution if you have a hole larger than your fastener that you need to seal. Reuse them a handful of times, as long as the material isn't damaged.

Galvanized Steel and Neoprene — Combine the strength of steel with the rust resistance of stainless steel. These washers are an economical alternative to 18-8 stainless steel in wet and outdoor environments. However, they typically have a shorter lifespan since the galvanized coating wears off over time.