I use "Goop", whether it is marine, automotive, etc.
Basically it is a silicone (from what I can find online) "RTV", with a thickener. Letting it, or similar, set and then cover (silver/aluminum) tape can extend the life.
Good news, it won't flow all over. It is also cheap from local sources.
Bad news, not much lasts long out in the sun or elements.

Silicone does not take paint well, thus using some sort of tape to cover the seal over time. Latex sealers take paint better, but in my eyes won't last as long even if painted.