Clean tubing above the scrape so tape will stick.
Soak a clean piece of small cloth or paper towel with Evaporust. Should be really soaked.
Wrap Evaporust rag around tubing tightly.
Wrap saran wrap or similar over rag.
Wrap saran with tape.
Might have to dry exposed tubing to get a good seal where saran meets tubing.
Taped saran should prevent evaporation.
Let sit for at least 24-36 hours.
Might need second application.
** If you can leave bike indoors if cold where you live as frozen Evaporust won't work.
Ps - clean off Evaporust before painting, don't just let tubing air dry.
Could always try Barkeepers Friend first directly on rust spot, though wash thoroughly before using above Evaporust method as Barkeepers can discolour paint if left on too long.
Too much messy work, and the possibility of paint discolouration.
Arrange the frame so the offending spot is horizontal, make a dam of Plasticine around your spot, and put a teaspoon or so of your favourite rust remover in the dam. Come back tomorrow.