Thinking that polyurethane would waterproof the sill, I sanded out the water stain, restained the sill, then coated it with polyurethane.
But the next winter, the thawed water still discolored the wood stain in the sill.
Is there something other than polyurethane that could create a thicker waterproof coating on the window sill and seal the joint between the window frame and the sill?
I know turning up the heat or using a dehumidifier could solve the problem, but I'm looking for a cheaper (long term) solution. Replacing the windows with vinyl frame is too expensive, however.