Lacquer Finish for Timbers with High Moisture Content

      Advice on finishing large timbers that may not be completely dry. March 28, 2010

I have a bunch of pine timbers to be used in a house that require a natural non-yellowing finish. I've been reading up on Sherwin Williams products because that's what's available in my area, and I've used their standard NC lacquer for years. Their CAB-acrylic lacquer seems to have good non-yellowing qualities, but some on the site have reported problems with CAB and moisture issues. The thing is, these timbers have about 12-14% moisture near the surface. What can I use that will meet my needs, considering higher moisture level in the wood?

Forum Responses
(Finishing Forum)
From contributor D:
Start with vinyl sealer. CAB acrylic lacquer over a vinyl sealer actually meets AWI requirements for water resistance, where the CAB acrylic won't meet them on its own. The vinyl sealer will also preserve the non-yellowing properties of the CAB acrylic. That said, I have no idea about water resistance coming from underneath.

From contributor B:
Vinyl sealer will not preserve the non-yellowing properties of the CAB. It may, however, improve the moisture resistance of the finish system. CABs are well known for moisture resistance that is not much better than plain old lacquer, but they are very yellowing resistant (notice I did not say non-yellowing).

With a moisture content of 12 to 14% I would be more concerned about checking than I would yellowing. If you finish the wood while it's at 12 to 14% moisture content and the wood begins to dry out, the coating may (or may not) flex as the wood shrinks. CABs do have good flexibility, but I think for what you are doing, a vinyl sealer will not help, nor will it hurt the overall system. Where are these timbers going to be installed?

From contributor G:
You are going to need a flexible, breathable finish. Flexible because, as pointed out, you don't want it popping off when the wood dries and shrinks. Breathable because you don't want it flaking off as the water leaves the wood. Perhaps one of the 2-part polyurethanes.

From contributor A:
These timbers are going to be exposed structural beams in a home. They already have some checking, and that's understood to be part of the character. I'm just looking for some sort of sprayable finish with minimal yellowing that will have a satin finish. What are my options as far as a breathable, flexible finish in addition to the 2 part poly?

From contributor R:
You might consider a CAB lacquer, as it meets most of your requirements. It's sprayable, available in a satin, and offers a pretty good record for a non-yellowing coating. It's also meant for interior use.

Your last request for a breathable coating is a misnomer, as a coating won't breathe. The wood beneath the coating will expand and contract, but as far as a coating that breathes... I've never heard of one.

An oil based varnish will have some give and take to it, and may absorb some of the expansion and contraction that the timbers will certainly experience, but you give up the non-yellowing factor that you requested. If you're set against using a 2K poly, the CAB acrylic coating might be the way to go.

From contributor T:
I believe you'll find that no finish will prevent moisture exchange between wood and the atmosphere. That is: virtually all finishes breathe. So don't worry about trying to stop moisture exchange. You can't - let it breathe.

Long oil (marine) varnishes breathe but are very flexible to accommodate the expansion/contraction that occurs in wood as its moisture content changes. That said, any varnish made with linseed oil is going to yellow over time. A long oil varnish made with tung oil will not yellow as much but they really aren't very sprayable and are usually glossy.

So stick with a non-yellowing, flexible finish and minimize the dry film thickness.

From contributor U:
I used SW CAB acrylic on a kitchen a few years ago. All the surfaces that did not get sunlight (inside doors, etc.) turned a greenish yellow. SW did not stand behind their product. And of course I never used it again. Had to repaint the whole kitchen.

From contributor R:
If you are going to use a film forming finish, send the timbers to someone with a kiln and have them dried to at least 10%. (You are close, but I wouldn't risk it. Refinishing timbers after they are installed is never fun.) If you don't, there is a real good chance of coating failure. I have actually seen finishes on timbers grow blisters full of water. Pretty cool looking but very unsatisfactory to the customer. If you can't get them dried, then an oil finish like Watco may work, or you could just wax them like they do in Europe.

From the original questioner:
I guess I'm going to go with the CAB lacquer over sand sealer. SW offers 5 or so sealers, so picking one is another decision. If it matters, I live in the deep south where humidity is high. I get 6% kiln dried lumber from the mill, and it's 8-9% within a couple weeks.

From contributor I:
8-9% is a lot different than 12-14%. Sherwin Williams spec sheet for CAB acrylic lacquer and also the one for whatever sealer you will use will have a spec for maximum MC. Just know they will not warranty their product if you exceed it and it will be one of the first things a lab will check. Maybe just to cover your butt, you should do a sample on the wettest piece you can find and see what happens.

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