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BradG 06-13-2008 11:23 AM

Best protective finish for old barn-board, used as table?
I want to build a kitchen table using these old, yellow pine barn boards. I'd like them to look like they did when they were wet with water. Bring out a little bit of grain, maybe slightly darker but I don't really want to stain them. Something to protect the wood from splintering and picking up stains.

If these boards, or post, look familiar, it's because I had been considering an epoxy type finish like Envirotex, in an earlier thread. I've basically dismissed that idea. I don't think the aesthetic holds true to the material.

slickshift 06-13-2008 10:02 PM

Y'ever used a Marine Varnish?
Not the easiest, but a great look

Also, an oil finish may prove interesting
Tung oil (100%...not the Tung based or Tung type oil)
Take a couple or 5 coats but a unique look for sure

chrisn 06-14-2008 05:44 AM

If you can get a quart of Draw tite,I would try that. I use it on my wallpaper table and it works great.

Nestor_Kelebay 06-17-2008 01:42 AM

The BEST finish for any working surface would be the hardest one. The harder the surface, the less it's going to be damaged and dirt embedded into it by sliding hard things (like stacks of dishes or stainless steel pots across it. So, in my view, the best finish would be a clear epoxy coating, or a clear moisture cure polyurethane (like the one Wasser makes).

However, if you're wanting a diy-friendly coating, then I'd stick to a regular alkyd based polyurethane or a very hard acrylic (water based) like Flecto Varathane Diamond Elite finish. The Diamond Elite is a cross linking acrylic and will dry to a pretty hard surface that should stand up reasonably well on a working surface like a table. A more durable finish would be an alkyd based polyurethane, but don't just buy any alkyd based varnish; get a quart (or gallon) of polyurethane hardwood floor finish, and paint that on your table (thinning it to prevent brush strokes).

A hardwood floor polyurethane will dry to a harder film than anything meant to be used on furniture.

BradG 06-17-2008 11:40 AM


Originally Posted by Nestor_Kelebay (Post 131025)
A more durable finish would be an alkyd based polyurethane, but don't just buy any alkyd based varnish; get a quart (or gallon) of polyurethane hardwood floor finish, and paint that on your table (thinning it to prevent brush strokes).

I'm in Manhattan, but I've got a balcony that this will fit on. If I wanted to use an alkyd floor finish, thinned as you suggest, how many coats do you think I'd need to apply and how long would it take to dry? Do you think doing it out of doors, on the balcony would work or risk too much dust?

Nestor_Kelebay 06-17-2008 09:40 PM

hang on. I typed the rest of this post before I realized you wanted to put this table on your balcony. If you want to keep this table on the balcony, then I'd suggest you use a spar varnish which will stretch and shrink more with the wood as it's moisture content changes due to seasonal changes in humidity. Either that or a softer water based acrylic finish like Minwax's Polycrylic. Alternatively, you could ask for a "deep tint" exterior oil based paint in a high gloss, and don't tint it at all so that it dries almost clear. You'd effectively have an exterior "varnish".

However, if you merely want to paint this thing outdoors and then bring it in, read on...

Basically, when you thin any coating, the thinner you apply normally evaporates completely (with the exception of using turpentine to thin old linseed oil based paints and varnishes). So, if you thin an alkyd paint with 10 percent mineral spirits, the dry film thickness will be 90 percent of what it would be if you hadn't thinned it. That 10 percent will be plenty enough to increase the drying time of the paint so that it self levels better to reduce brush strokes in your paint, but if you paint with a roller sleeve that shouldn't matter anyway.

I would apply two or three unthinned coats or three or four thinned ones. Just keep your brush or roller sleeve wrapped in a plastic bag (with as little air in it as possible) and that will prevent the paint in the brush bristles or roller sleeve from drying while the paint on the balcony cures, allowing you to do the whole job without washing out the brush four times and/or using the same roller sleeve.

Modern alkyd paints consist of "clumps" of drying oil molecules (like linseed or Tung oil) or parts there-of all glued together with a synthetic resin (most commonly consisting of phthalic anhydride and glycerol). The difference between alkyd paint and the old "drying oil" paints is that the oil molecules (or pieces thereof) are much more highly reactive with oxygen in the air. It's that reaction with Oxygen molecules in the air that connect "unsaturated sites" on the fragments of oil molecules in the alkyd resins, making the paint transform from a liquid to a solid. And, since the alkyd resins are much more highly reactive, alkyd paint cures to form a much harder film much faster than the old oil based paints did. Typically you had to wait 2 to 3 days for linseed oil based paint to be dry to the touch, and nowadays an alkyd paint is dry to the touch in 2 hours. If you're painting outdoors on a warm sunny day with a slight breeze, it'll be dry to the touch in an hour, allowing you to easily put 4 coats on in an 8 hour day. So, dust won't be much of a problem because the paint dries much faster than the old oil based paints did. Bugs landing on the paint while it's still sticky might be tho, but you can just pull them out after the paint is dry to the touch.

NOW, if it wuz me, I wouldn't put that many coats on that fast. I'd try to put a coat on every 3 or 4 hours, or as soon as you can walk on it without fear of damaging the paint. You see, when you put a coat of alkyd paint down, the first thing it does is absorb oxygen from the air. That oxygen then forms crosslinks within and between the alkyd resins, results in the paint film curing to form a solid. If you put another coat on while it's still curing, then you get good crosslinking between each coat of paint as well, and that ensures excellent adhesion between coats.

Also, and alkyd based polyurethane is not really an alkyd resin. They make polyurethane resins by adding di- or tri-isocyanates when making alkyd resins. When such a beast comes into contact with anything that contains a hydroxyl (-OH) group, the two react to form a urethane linkage. Glycerine has three hydroxyl groups, so those isocyanates form urethane linkages right inside the alkyd resins which behave very much like the roll cage inside a racing car. Those urethane linkages make the alkyd resin harder to crush if you could squeeze one, or harder to stretch if you could pull on one. THAT is the reason why alkyd based polyurethanes (which most people call "polyurethane") form harder and more durable films than alkyd paints. There are many different kinds of polyurethane chemistries, so I use the term "alkyd based polyurethane" when I'm talking about a polyurethane resin that's made from an alkyd resin.

That is, if you use a "polyurethane" floor finish on your table top, you will get a floor finish that's just as hard and durable as a hardwood floor finish.

Nestor_Kelebay 06-17-2008 09:55 PM

Because they have about the same "iodine value", alkyds and "polyurethanes" are about equally reactive with oxygen, so they dry to the touch in about the same time. It's just that the steel bars inside the polyurethanes mean that polyurethane films dry to a stronger solid. You get a harder film in about the same drying time.

The more you thin either an alkyd or a polyurethane, the longer it will take for the thinner to evaporate. However, once it's evaporated, the curing rate will be about the same for both.

So, if this table will spend it's life indoors, I'd probably use a hardwood floor finish on it if you don't mind the yellowist tinge it will add. If you want to avoid that yellowish tinge, you pretty well need to go with an acrylic (water based) coating.

BradG 06-18-2008 09:41 AM

Thanks Nestor. You certainly went above and beyond the call of duty with that explanation and I really appreciate it. I definitely will use a floor finish as you advise, that sounds like a great idea. Whether alkyd or acrylic, I've yet to decide.

And yeah, the table is for indoor use, I was just saying that I had the balcony on which to paint it.

One last question... what about the bottom of the table? These are pretty old barn boards so I imagine sealing them is a good idea. Can I just put one coat on the bottom first, or would something else be better down there, or do I not need to worry about the bottom?

Nestor_Kelebay 06-18-2008 12:30 PM

What you do on the other surfaces of those boards is just aesthetic IF you're planning to keep the table indoors. So, coat the ends and bottoms of the boards if YOU want. Doing so would be a purely aesthetic thing.

If you were to change your mind and eventually want to keep the table outdoors, then I'd recommend using a softer coating on it's top surface (like an exterior oil based paint) or a water based coating and painting all surfaces that you can on every board.

Outdoors, wood absorbs and expells moisture to the air around it, and changes in it's moisture content due to seasonal changes in humidity causes the wood to swell and shrink, especially across it's grain.

Hardness and rigidity go hand in hand. Since hard coatings (like oil based coatings which will naturally dry to a hard film) simply can't stretch and shrink as far as wood does, and that causes them to crack and peel off if you use them on wood outdoors.

To avoid that problem, exterior oil based paints are simply paints that are made with alkyd resins that are less reactive with the air, and therefore crosslink less, and therefore dry to a softer and more elastic film that will stretch and shrink with the wood.

Water based paints are naturally softer and both interior and exterior latex paints will stretch and shrink with wood outdoors. The difference between interior and exterior latex paints involves the amount and kind of additives (like mildewcides an UV blockers) used in them and the pigments (the white pigment frequently).

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