"What the consequences are for not waiting for oil on canvas to dry for several months before varnishing? The paint is thin but I need to varnish three weeks after completion." -- Rayart
Unfortunately, if you varnish an oil painting that soon, it will crack later. This because the varnish will dry before the paint does, and then when the paint moves as it dries it'll "break" the layer of varnish.
If the paint is thin, the cracks will tend to be tiny and will take about a year to appear, but it's not worth the risk because your long-term reputation may be damaged. Oil paint should be given at least six months to dry, more if the paint is thick and depending on the medium you used.
In the interim, use a retouch varnish to protect the painting (it can be applied when the painting is "touch dry") and offer to varnish the painting if a buyer returns it at some point in the future. Or they may be able to have it done at a good framer's or art restorer.
