To do it well you need to separate the fitting, clean and caulk. From the outside you are just putting on a bandaid. You can hold the mast
with a halyard
I'd use a flexible like Lifecaulk or etc.
I have done many fittings with 5200. My attitude: "do it once , never do it again" Caveat: Let the next guy try to get it off! haha
For some reason I dont like 4200 that well, but haven't tried it in 20 years (because of that)