Top coat

[Maple47]

Originally Posted by rabbit33

Well, first of all, pine will never really match oak; they're woods of drastically different types. Second, new pine will darken a lot in the first few years whereas your old oak has already done its darkening. So you need to go lighter with stain than you think. If you go with a dark stain on pine you end up with a blackish muddy mess.

Then, of course you need to put varnish over it. Use polyurethane spar varnish inside and out. (On the inside of a window is practically exterior exposure in terms of sunlight exposure, temperature extremes anyway.) Spar varnish has a certain amount of flexibility by intention. This means it'll never get really super hard or mirror glossy (which is why it's not a great choice for a tabletop) but it will flex under thermal cycling (which windows see a tremendous amount of). The name "Spar" implies its original purpose;
the spars and masts of sail boats. Heavy outdoor exposure, mirror finish not needed. You'll probably want two coats.

Pore sealers are irrelevant to this application. You're trying to protect windows, not make a mirror finish table top.

Shellac is a wonderful finish for the applications for which it works. I use shellac a lot. But NOT for window sashes! As I noted, for all intents and purposes you can consider the inside of window sash to be exterior exposure.

So, to summarize:

1) A lighter-colored stain than you would think.
2) Polyurethane spar varnish inside and out, probably two coats. (of course if the exterior's going to be painted a color, then use best practices there - a high quality oil base primer, then two coats of a high quality oil base exterior paint)

"Primer and paint in one" is probably fine for a child's art project, but not for things you actually want to hold up.

Thank you. Do I need to use a wood conditioner before staining?

[Seguinite]

Originally Posted by rabbit33

... "Primer and paint in one" is probably fine for a child's art project, but not for things you actually want to hold up.

You want to explain that to the folks at Behr, Sherwin Williams, and Pittsburgh Paints... all of whom sell it.

[Maple47]

Do I need to sand the stained wood before using polyurethane finish?

[Seguinite]

Originally Posted by Maple47

Do I need to sand the stained wood before using polyurethane finish?

0000 steel wool instead of sandpaper.