Hi Bill,
I'm afraid I have to disagree with the other responses.
Yes, you should most definitely use a sealer under the rubber strip, according to the TR4/4A workshop manual (same windshield/frame on all TRs 4-6) and from my own experiences. If not, you will definitely eventually see rust develop under there. Moisture can wick it's way back under the dash cover and eventually rot out the entire upper dash surface.
Even if your car is "sunshine only", sealing these places is important to keep out water when washing it, or should you ever get caught out in a rain shower. Think about it... No matter how good a seal you get with rubber only, even a tiny bit of moisture left along these seams will be forced further into the joint when driving, just by air pressure. Once under there, it's trapped and goes to work.
Use a non-hardening, thick sealer, usually available in tubes like caulking compounds or possibly in a long "rope" coil (the latter might be too thick for this job, though). It doesn't need to be paintable (like it does elsewhere on the car where it's covered with the final paint). The leading edge of the rubber gasket is what's most important, along with either end. Look closely and on most cars today, you will find a black rubbery sealer used at the ends of any rubber sealing strips.
Also seal the underside of the w'shield frame to that base gasket very well so that water can't get under the rubber there, either.
The trim piece on top of the windshield frame gets sealer under it, too. The type I used there in 1979 or so was the hardening type, and I can tell you that was a mistake!
And, seal well around the windshield gasket itself, where it fits into the frame. Again, I made a mistake not paying enough attention to this and can assure you from experience that water will get inside, be trapped and rust will start eventually even if the windshield frame is very well painted. This is particularly true of the lower, outer corners and bottom edge of the windshield, where moisture can pool.
You can get these sealers at a good auto body supply shop. Talk with them and get their advice as to what brand to use. The workshop manuals aren't much help with respect to brand, since they list common names that were in use in England 40 years ago, not the modern stuff we have to work with.
One other suggestion regarding mounting the windshield, put a some sort of anti-sieze (copper-based would probably work well) on the "pins" that go into the windshield mounting brackets on either side. This will make disassembly a *whole lot* easier years down the road, should it ever be needed. I don't think these parts were originally painted before assembly, so they tend to sieze up making windshield frame removal a real pain in the arse. The chrome screws that hold the windshield in place can benefit from a little anti-sieze or low-strength Locktite to protect the threads, too.
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