There are a couple of choices when it comes to window sealer. The Custom Shop is using R-900 windshield sealer from ADCO Products, available from most auto body supply stores. R-900 is a permanently flexible sealer that expands and contracts with temperature changes for a leak-proof joint. You can also use 3M Windo-Weld Resealant (No. 08634), which is more like glue and less like a sealant, or 3M Fast 'n Firm sealer (No. 08505). Apply sealer at contact points around the window.