Finish for window sash and sills?

[Kevin Begos]

I'm making sash windows out of old-growth cypress, and wonder about the best finish for those, and for the sills. I'm in N. Florida - extreme heat and humidity.
Had been thinking of Poly until I read another post on this forum that suggested:

"Varnish made with Tung oil produces a varnish that is better at waterproofing and better at stopping water-vapor." (as opposed to Poly)

I see some others are suggesting true marine varnish for outdoor furniture. Should I just buy one of those, or mix my own, and if so, what proportions or specific brands? Not planning on painting the sash, btw.
Thanks for any suggestions.

[Steve Schoene]

Are these double pane windows, and what exposure do they have?

[Kevin Begos]

Just old-fashioned single pane.
Mostly worried about the bottom rail and the outside sill, where the heaviest water flow is.

[Neal Clayton]

the insides, if you want them to be naturally finished, can be anything but there's also an abrasion factor to consider there due to the contact with the inside jamb. a hard furniture type varnish or a penetrating type old style varnish/oil blend is the way to go.

poly is a bad idea, it builds a surface film that will tear loose easily with said abrasion. i use waterlox on the insides of stain grade windows. stain or shellac or dye/toner beneath the waterlox for color, personal preference for how you get the color you want. sun resistance isn't an issue on the insides, the glass will block enough of the sunlight that any interior finish will work. waterlox thinned by half is a good mix of the best of both worlds. the sashes will entirely absorb 3 or 4 coats giving you a good seal, and the successive coats will build a hard film that the abrasion between jambs and parting beads doesn't rub off right away.

as for staining the outside, if you must do so, a very dark deck stain type finish would be the way to go, and know that if you do that the maintenance schedule will be shorter and more critical. you'll have to use wooden strips to glaze the glass in place, and that will have to be sealed with clear exterior caulk. i would cope these glazing strips as well, not just miter them, taking care that the joints point down, so that water drains out, and i would pre-stain the insides of them. the edges of the jambs should also be unfinished, you can wax them if you want them to slide a bit smoother, that's it. anything that can stick is a no-no.

glazing putty and paint is a much more viable long term maintainable solution for the outsides. if you choose to paint the outsides...

it must be enamel paint, due to the abrasion between the sash and the parting beads/jambs. the outsides should be primed with oil primer, not a water based primer. you will need to use some sort of oil sealer to give the primer something to grab on to, cypress has a watery sap that doesn't play nice with oil primers. i use my leftover waterlox for that as well, although anything that could be called an oil varnish would work (even a mix of blo/spirits or blo/turp etc). the edges should be left unfinished, tape them off. with constant weather contact leaving the edges unfinished allows the sashes to reach a moisture content equilibrium so that they don't expand/contract as much with moisture changes, and also lets any water that does get through failing finishes down the line escape, preventing rot. for the same reason, bridle joints or through tenons are preferable for the joinery, rather than hidden mortises. on that same topic, you must use at least a water resistant PVA glue (such as TB3) or better yet, a poly glue (such as PL adhesive or gorilla glue) or an exterior epoxy. dap33 works ok for putty if you decide to paint, there's an acrylic glazing putty called glaze ease that also works, although it shrinks a bit due to being water based so the look isn't as clean as traditional putty, and for premium putty, sarco type M. the latter two are available in small quantities online. oil putties such as sarco and dap33 must 'skin over' for a week or two before painting. glaze ease can be painted the next day. there are no long term results on glaze ease yet, but i did some sashes with it 3 years ago and it's holding up ok, so as of now, i'm still using it. the paint should extend past the putty on to the glass by about 1/16 to seal the putty.

either way, the glass should also be bedded in clear caulk or glazing putty to seal the inside (you can and do get condensate on the insides of single pane windows).

the same principles apply to the sill, exterior parts of the frame, exterior casings, etc. oil primer, followed by acrylic enamel paint is the best way to go. even if you want the outsides of the sashes stained, there's no reason to stain the frame and sill, the extra protection of paint is even more critical on those. the sill should slope to the outside about 10-15 degrees from level (take note of any amount the wall is out of plumb, might need to be more to compensate) for water to flow away. the bottom rail in the lower sash can also be unfinished on the bottom, like the other edges. the general idea is allowing the wood to breathe is better than trying to over finish it and trapping water.

parts that touch the sashes (back edge of interior sill (stool), parting beads, etc.) should either be sealed/waxed but otherwise unfinished, or stained with something that completely penetrates. in the case of painted outsides, i thin the paint to the point that it becomes a stain and the strips soak it all up. anything that builds a film will just rub off.

you're looking at a 5-7 year maintenance cycle if you stain the outsides, at best, with a deck stain type exterior varnish.

with paint and putty you can extend that out to a 13-15 year paint/putty maintenance cycle.

all of the above is why traditional windows and doors cost so much. it's not the making of them, you can cut and assemble the parts in a day, it's the finishing that takes so long. when you must use so many oil based products for their weather resistance there's a lot of time spent sitting around the shop waiting on paint/putty/stain/sealer/whatever to cure before the next step.

on the plus side, you will have 100 year windows when you're done, rather than those 10 year plastic windows. so you get bonus points for that .

if you have design issues or questions, highly recommend the book "modern practical joinery" by george ellis. it has alot of traditional window sash/frame design examples, it was first published in 1908 i believe, so is authentic for historic restoration type work as well. its examples are a bit overkill in complexity in some instances due to them not having modern adhesives back then and having to defeat water intrusion by clever joinery alone, but the basic principles still apply.

[Kevin Begos]

Neal,
Thanks for all that detailed info.Exactly what I was looking for. Hadn't thought of the "sticking" issue but a very good point.
I'll certainly paint the outside sills, and perhaps the outside sash, too. The natural wood on the inside is the look I was more concerned with keeping.

Last q: I love the old-growth cypress (hauled up from local river bottoms) but also know a local sawmill that will do quarter sawn white oak. Would that be better for the sash? I know it's hard, just not sure how it stands up to the weather. The cypress stands up very well.
Thanks again for all the details.

[Neal Clayton]

white oak does cost a lot more, but is up there with mahogany in choices for premium exterior use hardwoods, it's almost impervious to water. think about it, they make whiskey barrels out of white oak, so it has to be liquid proof . i would say it's personal preference. both finish easily, with the exception of needing an oil sealer coat on the cypress to get an oil primer to stick properly.

the reason i use cypress, besides the lower cost, is the weight. white oak will be a lot heavier and will require heavier counter weights, if you are building counter weighted double hungs. fitting weights that large in behind the jambs can be an issue with sashes built from heavier hardwoods, since you have limited clearance.

a typical 4' x 3' cypress sash, finished with the glass in place, comes in around 16 pounds. white oak or mahogany could wind up being closer to 20-22.

the only other gotcha with white oak is you should use copper/bronze or stainless nails for the metal weather stripping and bronze for the weather stripping itself, if you go that route for the frames (or if you choose the V style weather stripping on the sash, rather than the flat type on the frames). the tannin in white oak will react with iron/steel fasteners and weather stripping.

[Kevin Begos]

Thank you for the tip on the joinery book. Found an electronic copy - amazing drawings and detail.

[Rich Engelhardt]

white oak does cost a lot more, but is up there with mahogany in choices for premium exterior use hardwoods,

What about Spanish Oak?
Any insights on using that?

BTW - excellent essay above. Spot on "text book meets real life", in every respect.

[Neal Clayton]

i've never used spanish oak, although i'm sure there are other species that work ok. generally anything that works well for an exterior door will work fine for a wooden window.

my range of travel is from arkansas through louisiana and mississippi so the most common species used for window construction/restoration around here are cypress and spanish cedar, with white oak and mahogany being used if a hardwood is preferred. in the old days when they had old growth longleaf pine that was used as well, but most of those trees are gone (at least the ones with > 10 growth rings per inch) so it isn't very common and is rather expensive when you do find it around here.

species with minimal movement between seasons due to moisture content are the better choices since you can make the check rail joints (the joint between two double hung windows where they meet and seal out drafts) very snug and thus create a better seal. on the above mentioned typical 4x3 double hung, i only get about 1/32 of movement in arkansas between summer and winter with cypress, so it works really well for that purpose.

here's a pic of one of my recent ones that's stain grade on the inside...

http://www.historichomeworks.com/for...oad.php?id=839

sashes are cypress, casings and head molding and sill and such were reproduced from longleaf pine to match the other original trim, all finished with garnet shellac and waterlox.

other things that come to mind with the cutting/assembly...

don't worry too much with the height and fit while you're cutting and putting the sashes together, each one will be a custom fit to the opening, just make the bottom rail about 4-5" wide and you can cut the sill slope and trim it to fit after the fact. added bonus for this is you wind up cutting your joints flush and don't have to worry about sanding them and glue cleanup as much. if you're doing double hungs and need that rope/chain groove for the weights, you can turn a tenon jig around backwards and clamp a fully assembled sash in the jig that way to run over a dado blade for the groove. you can also integrate the check rail on the lower sash by turning the top rail of the bottom sash on edge and making it wider. since the width will then be what shows you will have 1 3/8" 'reveal' on the meeting rails, so making the jambs and stiles in such a way to shoot for a 1.5" reveal will make them match (1/8" difference isn't visible). the check rail on the upper sash will just have to be nailed/glued on after assembly. there's no way to integrate it and cope the bottom rail since your check rail would run into the spindle on the shaper/router. the check rails should be snug to the parting beads that they slide inside of, so they also should be cut to fit after assembly, i just use a handsaw/chisel to knock out the bulk and file the ends to fit after that. i don't bother trying to make sloped check rails like they did in the old days, it's a pain to get them to fit together for not much benefit. back then they didn't have drafts to worry about since they didn't have forced heat and air, but that is a concern these days, so i just make them interlocking square strips, it's easier to keep them snug against the parting strips and stop drafts that way.