How long does it take for the sun to make it peel? This is usually not a normal thing. Sometimes moisture from the windows leaking onto the stained surfaces can make the poly peel sooner though. I have used varnishes, lacquer and poly and all have held up well for years of service. There has to be something else going on.

What PG said...

Poly just doesn't peel for no reason.
There has to be something else happnin'!
>>> It's almost alway related to moisture....

1) What type of windows are they?
1a) How old are the windows...?
1b) How well insulated is the window area?
1c) What was the original finish & wood-species?
2) Does the wood trim get frost running onto the wood?
3) Does just the bottom trim peel, or is it uniform around the whole window?
4) Are ya sanding between coats, when poly is hard enough? This takes many hours typically.
5) Exactly what brand/type/sheen are you using?

Faron

The windows are eighteen years old (anderson) but not scheduled for replacement. It is a yearly thing and moisture getting in is probably part of the problem. My neighbor suggested I used some sort of finish that is normally applied to boats. If this would work, what kind would I buy?

Are we talking inside our out?
What was on the wood originally? Did you clean and prep sand ? Acrylic poly over oil varnish is gonna peel.
CAsey

I guess I would say the windows are "outside" in that the trim around the windows faces outside and is exposed to the sun and other elements when opened or perhaps even closed (moisture). I used a Varathane stain and a plain polyurethane, satin, over that.

Yes, the product used on boats would be a marine varnish. Instead of this, I would use Sikkens Window and Door in satin. But these are generally products that are designed for exterior use. They take some time to dry and will stink up your house pretty good. I would take the windows out if possible. I think what you could be experiencing is trying to finish wood that is really too beaten down by the sun to hold the finish. Or you could be trying to finish over a finish that is not compatible with the one you are using (poly over shellac, poly over a water based coating, etc). But, overall, window sashes are a pain. Usually, they are not finished properly to begin with by the painters or the homeowners so it does not take the sun long to wear away that weak layer of finish and get to that stain layer. The sun coming through the glass has a sort of magnifying effect on the stained surfaces which will accelerate the time it takes for the finish to degrade. This can be made worse when moisture is involved....the frost from the glass can seep onto the horizontal areas and puddle there and then the sun comes along and cooks that moisture. This is why you see so many people with windows that have turned black and have developed water stains...it just doesn't take long.