heteroscedastic wrote:
I have Andersen double-hung windows. I finished the wood sash with
polyurethane 10 years ago. It is generally in fine shape except for
windows which experience a certain amount of condensation in the
winter, mainly early in the heating season before the house has dried
out. I wipe it off every morning but that's not enough, and the
varnish has deteriorated on the beveled part of the bottom of the
sashes. I think if I just paint the sash, the paint will hold up
better than the varnish has, though I don't expect perfection here. I
can't totally prevent condensation, mainly because I heat with wood,
and the bedrooms are cooler than the rest of the house.

Are there any tips for getting the best possible result with paint,
i.e. maximum resistance to damage from condensation?

I don't know your brand of window, but for wood windows in general the
problem is moisture running down the glass behind the wood. Only tip I
know of for interior window finish in regard to this problem is to paint
or varnish onto the glass - provides a seal so the water doesn't get
behind the wood. If your hand isn't steady, you can tape the glass and
allow 1/16" or so of glass to be finished. I have also used a metal
rule and razor blade to even the line after finish is dry.