On Tremclad, it's made by the same company. Rustoleum is harder to find in Canada and I haven't found the "Professional" grade here anywhere and I buy it the US where it's easy to find in Lowes, etc. I am not sure if Tremclad is formulated exactly the same as Rustoleum but would be surprised if it isn't the same and just re-branded. Either will be good for a TT frame.
If it were me, I'd brush it on to avoid getting overspray on the exterior and parts you don't want paint on, except maybe in visible areas like the A-frame or bumper and you can always mask it off with sheet plastic. You're not going to be showing it at a concourse event and keeping rust at bay is what matters. Nice thing about Tremclad is that if any rust reappears, just sand it down to remove loose rust, clean it and re-paint over it.