These grommets go though the outer panel only. Stainless steel grommets have a bright and shiny passivated finish as per AMS2700 which offers excellent corrosion resistance. Zinc-plated grommets are steel with a bright zinc and yellow (gold) chromate finish which offers good corrosion protection.
Sizing: Measure the thickness of the outer panel and select the shortest grommet that will fit it. Properly installed, the protrusion of the grommet on the back side of the panel is absorbed by the inner panel, support structure, receptacle or a combination of these. Therefore, no gap will exist between panels. Using a grommet that is too long may result in it jamming into the receptacle.