I installed windshield in my '69 myself since all the shops said they didnt want to work on old car and a host of other excuses. New gasket, zero sealant, zero leaks, not one drop of water gets in. There are drains in bottom of windshield channel, one on each side, so any water that gets in channel flows into cowl. Most of time those drains are filled with sealant from previous hack installs. So if you fill drains with sealant water can't drain which means better have a ton of sealant cause its gonna flow into car otherwise. I didn't have any rust in channel so that helps too.
