Primer
Primer turns up in 2 fields: adhesives, waterproofing. Here is what it means to the people in each one.
Primer in adhesives
A thin first coat wiped or sprayed onto a bonding surface so the adhesive grips better or the fresh prep stays clean.
Bonding primers range from solvent solutions that let cyanoacrylates bond polyolefins, to glass and paint primers for windshield urethanes, to corrosion-inhibiting primers that lock in a fresh aerospace aluminum surface until bonding. They are applied very thin, usually wiped or sprayed, and given a set flash time. Too thick a primer layer becomes the weak link and fails cohesively.
"Primer has to flash ten minutes before the urethane bead goes on, don't rush the glass."
Primer in waterproofing
A thin liquid brushed or wiped onto a joint face or deck so the sealant or membrane bonds reliably.
Sealant primers are matched to the sealant and substrate: some seal dusty, porous concrete and masonry, others chemically prepare metals, coatings or plastics. Membrane primers consolidate the surface and improve tack for self-adhered sheets. Primers have a set open window: applied too thick, left too long or allowed to get dusty, they become a weak layer instead of a bridge. Pull tests confirm the choice.
"Primer on the precast faces only, don't slop it on the glass, and seal within the hour."