The seams on my 1958 TR3A were all tightly secured with a black tar-like substance that the Brits call "Dum-Dum". Why they call it that, I can't say. Maybe that's the name of the worker who had to do it. My earliest experience with this was about 1962 when I removed the tunnel over the gearbox. I pulled and tugged and pried it till it came out. Then I wiped it all off with a putty knife and followed by a solvent. When I dismantled my car for a total body-off restoration from 1987 to 1990, I had to do the same for the four fenders. What a job.
In the last 18 years, i have never used any sealer in any of the gaps. All the parts were pre-painted off the car, so all the flat seams are pre-painted face to face. During the last 18 years, I've never had any paint blisters or bubbling in 100,000 miles since then.