I use LEXEL on the hard seams and joints and marine silicone on all the light fixtures. Reason...lexel dries sightly stiffer than silicone but still remains durable if you have to take things apart. The silicone seals the lights and can be removed when changing bulbs without to much hassle.
LEXEL is tough to work with and clean up so patience works (small areas)...that and a wet finger, but be quick because it skins over rapidly and drags. If you are good with a tube you won't have to do anything but apply.
Jay and Linda