When I did my barrels in situ, advice here was to use saran wrap and a length of string to wrap the head/barrel interface, then fold saran wrap up over head to make a near perfect protection of head and a clean line to barrel. No tape in that area needed.
I used high temp 1200 F) bbq paint, satin black, brush on, as spraying in my public parking garage work area not an option.
Worked out very well. Directions stated to just remove loose rust, no need to fully de-rust. And only one coat needed. Used a foam wedge "brush' for some of fin gaps until the foam broke away. Then went with pipe cleaners dipped in paint. Worked quite well.
Has held up nicely in two years so far.