SCREEN MESH
In our smaller vent caps under 3 inches, the screen meshes we use are 24 mesh or finer to exclude insects such as mosquitoes. The mesh is plastic so it it non-corroding, however there are different choice of material available. Conventionally we use polypropylene for its physical rigidity but Nylon and Polyester are also viable alternatives. Here is a chemical resistance chart comparing the three materials side-by-side.