One of the most important steps in making a choice of product is to seek advice from the manufacturer and use the"system" that is advised.
Buytal mastic is perfect to use as a gasket
but is not good fully immersed in water
it s properties are similar to glazing putty but perfect if used correctly.
Most silicones used correctly will require a primer for some substrates and more so when used as a structural adhesive
, but lots are not used as intended as the manufacturer suggests.
Polyurethanes are similar to the others and require certain processes to apply them .
If all steps are followed most products will do what the manufacturer states they will do.
I see many failures every day and 99% are operator error.
Seek out a range of products you are comfortable with and get the manufacturers web site to gain as much knowledge as possible on the few products they suggest for your application, if your unsure ask them NOT the shop assistant for a recommendation.
Pick a large manufacturer of sealants in your area like GE,Sika,Mapie or BASF