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imhotep
Vendor - Optical Supports
Reged: 02/14/07
Posts: 1485
Loc: Lutz, FL
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In a couple of weeks I'll be ready to stain and seal a truss-dob built out of baltic birch HVHC plywood. What is recommended as a good exterior-grade varnish or finish?
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Curt
www.opticalsupports.com
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cloudmagnet
member
Reged: 12/04/08
Posts: 47
Loc: Georgetown, Texas
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I would suggest that you reconsider staining BB ply- birch is a very dense wood that doesn't accept stain well. At least try a test sample on a piece of scrap. As for finish, try MinnWax poly water-based finish.
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mtb54703
sage
Reged: 11/12/08
Posts: 259
Loc: Eau Claire, WI
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Stain can be tricky on that material, very blotchy sometimes - if you want to stain, be sure to use a pre-stain condition on the Baltic birch. And try it first on sample/scrap pieces. I've also had good luck with the gel based stains in these situations. My recommendation would be to skip staining and just apply a finish.
As for a finish - water based stuff is user friendly, dries quickly, and is easy to clean up. I've used it on a couple of projects and don't particularly care for it. I'd recommend Minwax Spar Urethane - 2 or 3 coats. Comes in a satin, semi-gloss, and gloss. For your project I'd thin the first coat 1:1 to better penetrate and seal the wood. I get my best results with a foam brush - it goes on a bit thinner and doesn't have as much of a tendency to run or sag - but then I'd probably do 3 coats. Sand lightly between coats. I wish I could remember what line of 3M sand paper I recently tried - it was pricy - about $2/sheet, but it didn't clog up. Excellent stuff.
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Ray4852
professor emeritus
Reged: 09/30/08
Posts: 550
Loc: Arcade, NY
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I finished my home built scope with marine spar varnish. I used birch veneer plywood. no stain apply the varnish over the plywood. its a little work putting it on but its the best finish you can use to protect a scope. my second choice is polyurethane. this stuff goes on easy but wont hold up as well as marine varnish. its not a good idea to apply stain to plywood. the stain will run and wont dry because of the glue in the plywood. apply the varnish with light coats or it will run. let it dry for a couple days then sand it lightly then apply another coat of varnish sand it again and put on the 3rd coat to finish the job. you have choices of finishes you can use. I like satin. you can use gloss. satin is better. gloss will show every flaw in your work.
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Home Built 18 dobsonian
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Spoonsize
Vendor/Clothing
Reged: 08/27/04
Posts: 3334
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A white sandable primer, automotive primer topped off with automotive enamel.
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3963
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I utilize polyurethane varnish- thinned with mineral spirits (first sealer coat 2 parts m/s, 1 part poly, the remainder 50-50). I scrub as much as I can into the wood with a rag- then wipe off as much as I can. Let dry 24 hrs between coats, lightly sand with a fine grit paper just to knock the whiskers and dust bumps off. 3-4 coats applied this way will give a beautiful finish- and you can invisibly touch it up any time by simply scrubbing another coat on/scrub it off. No brushes required, rags only- so no cleaning of brushes.
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Wes
Atlantic Beach, FL
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5211
Loc: Mobile, Alabama, USA
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Use exterior grade polyurethane, which is also known as "spar varnish." Marine grade polyurethane will also work well. Do not use interior grade polyurethane such as the kind used for floors, it's not meant to cope with the wood expanding and contracting from going to a warm house to freezing weather than back again. Exterior grade polyurethane is made with that in mind, and when applied properly is very durable and resistant to the acid dew that is everywhere.
Taras
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15-inch F/4.5 Dob under construction
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rboe
Reged: 03/16/02
Posts: 47546
Loc: Phx, AZ
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I am fast becoming a big fan of Benite, a product out of Seattle from the good folks at Dalys Paint (dalyspaint.com).
Couple coats of Benite (or more), let dry and Bob's your uncle. Acts as stain precoat so stain goes on evenly and for good measure it hardens the wood.
Tends to darken the wood a bit (a positive for cherry and mahogany - birch not so much - that is, it doesn't darken it much).
Fisch did a very good article on testing different exterior varnishes. A good read for just this.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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imhotep
Vendor - Optical Supports
Reged: 02/14/07
Posts: 1485
Loc: Lutz, FL
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Thanks everyone for the feedback. I think you all collectively set a record here for being the most in agreement on a ATM question 
Marine spar varnish it is! Last year I built an entertainment center out of high quality maple plywood. I used Minwax water-based PE to seal it. It went on with a synthetic brush and I wasn't happy with it as an applicant, so I'll take the advice and use a foam brush or rag this time.
The staining didn't strike me as problematic with that maple plywood, but if baltic birch is much harder then I'll take your word for it. The result pleased me but I don't have a lot to compare it with in the past. It was a little difficult to sense when the stain had penetrated fully. If baltic birch is that much different I can see where it would be a problem.
This is a little disappointing as I'd gotten my heart set on a dark stain with brass hardware. At the very least I can go the route of no stain with 3-4 coats of marine spar varnish.
But with that said, I'd be interested to hear from anyone who's got a good reliable system for staining baltic birch. It has to be possible, wise or not I won't be stubborn if it's flirting with a ruined pile of wood, but having a dark stain is really what I want.
Thanks again.
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Curt
www.opticalsupports.com
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3963
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Curt-
If you really want to stain it, you can... it's simply that the baltic birch ply is very absorbent, and as a result, if you try to apply stain to the wood bare, it will soak it up unevenly, resulting in a blotchy job. The trick is to seal the wood first- or pre-treat it by sealing it with a coat of some kind of sealant. A thinned coat or two (preferably 2) of the mix I mentioned of (1st coat) 2 parts mineral spirits to one part of poly, let it sit 24 hours, scuff sand it- apply a 2nd thinned 50-50 will result in a good sealant for the stain. Here is a pic of one of a pair of cabinets I built/finished utilizing this very method. Birch Plywood, Maple solids, cherry stain. About 3 coats on top of the stain, applied with a rag- 50-50 mix. Scrubbed as much on as I could, then scrubbed as much off as I could. Still a bit of blotching of the stain- these cabinets were a learning experience...
Wes
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Gordon Rayner
Pooh-Bah
Reged: 03/24/07
Posts: 1096
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I am no expert, and have not tried staining, but have tried various varnishes on birch plywood ,red oak, eurobeech, pine, and douglas fir. If you are near a West Marine, they sell some varnishes for the demanding boat environment. I have had more durability with those than the Minwax and others,such as found at Home Depot or Lowe's, for example, including the air-dry polyurethanes.
The two part marine polyurethanes are expensive. The one-part Sikkens Cetol Marine Light is very tough, but requires buffing with the 3M abrasive loaded nylon pads between coats. I now favor the Epifanes spar varnish. Epifanes makes the WestMarine house brand spar varnish, which is slightly lower priced. I use it. It occasionally is sold at seasonal special prices. The first coats should be thinned.
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imhotep
Vendor - Optical Supports
Reged: 02/14/07
Posts: 1485
Loc: Lutz, FL
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Quote:
Curt-
If you really want to stain it, you can... it's simply that the baltic birch ply is very absorbent, and as a result, if you try to apply stain to the wood bare, it will soak it up unevenly, resulting in a blotchy job. The trick is to seal the wood first- or pre-treat it by sealing it with a coat of some kind of sealant. A thinned coat or two (preferably 2) of the mix I mentioned of (1st coat) 2 parts mineral spirits to one part of poly, let it sit 24 hours, scuff sand it- apply a 2nd thinned 50-50 will result in a good sealant for the stain. Here is a pic of one of a pair of cabinets I built/finished utilizing this very method. Birch Plywood, Maple solids, cherry stain. About 3 coats on top of the stain, applied with a rag- 50-50 mix. Scrubbed as much on as I could, then scrubbed as much off as I could. Still a bit of blotching of the stain- these cabinets were a learning experience...
Wes
Thanks for your help Wes. So when you seal wood before staining it, you are basically creating a shallow barrier in the wood? Just trying to wrap my head around what actually happens. In other words the stain still penetrates the wood but only slightly compared to untreated wood?
I guess I'll think on this before making a decision. A friend of mine may be able to put on the finish for me in his spray booth. Initially I wanted to do this for the time and effort savings, leaving only a stain job for me to do prior to finishing in the booth. But if I have to pretreat two coats, then stain.....I may just leave it natural as others have suggested. Bah, too many choices!
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Curt
www.opticalsupports.com
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rboe
Reged: 03/16/02
Posts: 47546
Loc: Phx, AZ
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Another choice that may work for you is to get pre-glued veneer. Comes in very thin sheets and is applied with an iron. Then you have the colour of wood you want.
One advantage to veneering is that you end up with the wood grain you like. Birch can be pretty boring.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3963
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Quote:
So when you seal wood before staining it, you are basically creating a shallow barrier in the wood? Just trying to wrap my head around what actually happens. In other words the stain still penetrates the wood but only slightly compared to untreated wood?
Yes... most importantly, it helps maintain an even penetration, so you don't have blotchy patches where it stains darker- due to more penetration- and lighter areas- where the stain doesn't penetrate as deep. Soft woods act like a sponge and really soak up the stain. The idea behind staining is a controlled coloring/shading, something you won't achieve if the wood acts like a sponge. You can't pull it back up/control it.
Wes
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RossSackett
scholastic sledgehammer
Reged: 08/17/07
Posts: 756
Loc: Memphis, TN
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I agree with the above reservations about staining Baltic birch, and I'll add another--BB often has parallel knife checks that act as little cracks that wick in stain, sometimes even if you use a conditioner. This can create a peculiar surface pattern of parallel lines, independent of the grain and pores of the wood. This pattern can become more pronounced over time.
For this reason I usually veneer BB with a dressier wood like mahogany, curly maple, or walnut. But when I need to use Baltic birch as my show-wood, I like to spray or rag on a subtle yellow-orange die stain to warm it up, with urethane varnish over that. The additional color evens out the differences in tone in the wood and mimics the slightly darker knife-checks, and to my eye makes them less noticible.
Ross
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
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Trondur
super member
Reged: 03/09/07
Posts: 113
Loc: Cumberland County, Maine USA
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I mostly use oak veneer plywood, but I use a process similar to Ross'. I stain with linseed oil, followed by as many coats of poly as my patience will allow.
I apply the linseed oil with a rag. On the oak it gives a nice light golden colour. I usually put on 2-3 coats. Be very careful with the used rags, they can ignite spontaneously! I keep mine outside and away from things until trash day.
For a poly finish I like building up coats of a gel clearcoat. Bartleys was the brand I used - I don't know if it's still available. I think Minwax has one also. I like it because it goes on with a rag and you wipe off the excess. No runs, bubbles, or dust. Very water-resistant, as well.
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Terry
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