you can ger clean lug brass brazing
it is about
use a big flame
not a small intense hot flame
you move quick
you flow from one point over a large area
spill over is because the lug is hotter than than the tube [it will flow first and fastest to the hottest area**
you have to get lug and tube to the same temp
and keep the large flame moving about
and get the angle of the rod so the flame just kisses it or a blob melts off onto your lug
practice and more time
"Comwelds brass and copper flux", redishpink stuff. Good Stuff, we have it down under and in the UK
In the old days they used just straight Boric acid fluxes, it was yuk.
Harris green stuff is good by all accounts by the pros in the USA
and there is no need for a gas fluxer blah blah.