What it is
Where the inner and outer body panels meet around the windshield opening, they are pinched together and welded — hence the name. That flange is painted, then a bead of polyurethane adhesive is laid on it, and the glass is set into the bead.
The bond between adhesive and flange is what holds your roof up in a rollover. It is doing structural work.
Why primer matters
Cutting the old windshield out leaves a thin layer of old urethane on the flange, and it scratches the paint. Fresh urethane bonds beautifully to a thin trimmed layer of old urethane and to primed metal, and poorly to bare scratched paint.
So the flange is cut back to a consistent thin layer, and any exposed metal is primed. Skipping the primer is invisible on the day and shows up in three weeks as a damp carpet.
Rust is the one that stops the job
If water has been getting past a previous bad installation, the flange corrodes underneath. Urethane will not bond to rust — it bonds to the rust, and the rust lets go of the car.
When we find it, we tell you before starting rather than gluing a windshield to a flange that cannot hold it. Treating it is body-shop work. It is also the single most common reason a leak comes back after being "fixed" twice.
Need this done rather than explained? There is no shop to drive to. We come to wherever your car already is — your driveway, your workplace car park, an apartment lot, or the side of the road it stopped on — anywhere in greater Sacramento, with no call-out fee and no mileage charge. Give us the year, make and model and you will have a firm price before the call ends.