Carpentry and Woodworking - refinishing windows

We have a 1953 home that has the original Andersen storm master double pane windows. Over the years the inside varnish finish has deteriorated from condensation and I have refinished them. The problem I have is a couple of the windows' where the wood at the glass have hairline separation cracks and the finish will not last. My guess is that air is coming in under the varnish and causing the failure

So my question is what would be the best product and procedure for sealing the cracks and stopping the air infiltration? Also, if others have a different theory for what is causing the failure than mine, please share your insights. Thanks

P.S. We have corrected the condensation problem so that shouldn't be a problem.

Can you further explain "the wood at the glass have hairline separation cracks and the finish will not last" ?

Often varnished wood doesn't get all the attention that it should at repaint time. Usually the biggest issue is getting the wood to look right from all the years of neglect [faded, water stained. etc] The cracks in the wood shouldn't cause issues with the life of the finish although by this time they often require extra coats of varnish to properly seal them. I usually use colored putty to fill in any cracks in the wood.

Can you further explain "the wood at the glass have hairline separation cracks and the finish will not last" ?

The cracks are in the trim piece where the glass meets the wood. The trim is about 5/8 " wide. Maybe my use of the word "hairline" is what is throwing you off. The cracks are small in width and run the from 6" to just under a foot in length. Do you thing putty would do the trick?

I also note that you think the cracks may not be what is causing the failure. What is your guess? And what would be the fix?

Thanks again.

The sun and moisture condensation are the main culprits on paint/varnish failure on windows. I'm sure you noticed the majority of damage/failure is restricted to the bottom part of each sash.The only real fix is to keep the coating in good shape by sanding and recoating. It may require more than one additional coat. I have stained woodwork in my master bed rm. 1 window sees a lot of sun and recently I had to sand that window sill and the bottom part of the top and bottom sash and recoat. The rest of the wood looks as good as the day it was poly'd - about 6-7 yrs ago.

If the crack is between the wood and the glass, it would be better use caulking. A clear latex caulk is fairly easy to use. It comes out milky in color but dries clear. Sometimes it will become milky again if it gets wet but will clear up again when it dries. If you aren't profficent with a caulking gun - take a wet rag/sponge to smooth out the caulk and then a dry rag to remove any excess residue.

And yes the windows with the problem are on the west side and get lots of afternoon sun. I guess this will be an annual/biannual project. The upside is the windows and the rest of the trim are in very good shape.