Having worked around auto glass and glue for roughly 30 yrs, here's some of my findings.
1) Soft, butyl based black sealer will stay pliable virtually forever. It will soften on warm days and spread deeper into the joints. It also has the advantage of wicking minute traces of petroleum into the pores of exposed metal, acting as a terrific rust inhibitor. It's available in rolls of many sizes (by Tremco & Essex) including 1/16 x 3/8 wide.It can be laid along a line and nails driven through so it squeezes out. Also put up in standard chalking tubes by CR Laurence company.
Any residue can be sheared off with a popcycle stick. Mild petroleum based solvent can be used to finish edges and butyl dries to a non tacky soft film in a few hours.
Shelf life is pretty much forever.
2) Urethane, when dry, has the consistancy of a rubber tire. If applied to a smooth clean surface it will either stick like S.T.A BLANKET...or not at all. Nothing in between...it works well or not at all.
Cleanup can be messy. You usually have a short time frame to work it, and if it "skins over" the adhesion becomes almost nill.
Urethane comes in 3 viscosities. The catalyst for urethane is moisture so wear gloves as it begins setting up in minutes on bare skin. Urethane has primarily been used to hold windshields in cars since the early 70's. Shelf life is a few months after opened.
I can't really think of anywhere I would use it on my Model A.
3) Silicone is great for bathtubs & boobs. It exudes an acid which promotes corrosion around bare metal at a fierce rate.
*PAINTED surfaces seem to be protected from it's evils, although it can stain paint and glass if water carries the acid down a surface and it dries.
When silicone is put around a vehicle's windshield (in areas which leak) the result is intense black rust...soon resulting in holes corroded right through the metal.
On RV's, which seem to ALL leak, silicone sticks for awhile to the fiberglass, giving a false feel of security and then breaks down & permits leaks, leaving the inner structure to rot from within.
Below is just some pictures I had of the pinchweld above the windshield, on a van which had the windshield "siliconed" a few years previously.
