A regular soft pencil lead has never failed me.
When mounting sights, I brighten up the metal, clamp the sight into position and then scribble all around the sight base with the pencil to cover the bbl & base with the lead (graphite).
Remove the sight base, tin the barrel area & bottom of the sight base (nothing will stick to the graphite coated steel surrounding it.
Put the sight back in place , clamp and sweat solder. It's quick and little or no solder clean up necessary.
You can do the same with the rib.
The soap stone works well too (same idea) I'm told though I've never tried it.