Sealer Coat Questions

[Benvolio]

So I was wondering, if a sealer coat really does seal the wood - how comes the wood still moves with moisture changes during the year??

I mean, if the wood is sealed, how is the internal moisture affected?

Just a thought....

Ben

[areynoldsre]

"sealer coat" is a misleading term. No finish, even sealer 100% blocks the movement of moisture through the finish and into the wood. Slows it down some but it is still there. and few things are coated 100% - there is always some bare wood some place or a chip or scratch for moisture to get into.

[Benvolio]

That makes sense that the sealer coat would be semi-permeable.

But what about pieces that are sealed on all faces witih, say, polyurathane - that surely must be moisture proof, but it's still advised to use, for example, a breadboard edge in case of movement.

Same question really - where does the movement come from???

[Beechwood Chip]

See above answer, and if you insist that the finish is completely impermeable, think, what about after a year? Five years? Ten years? Well designed wooden furniture can last centuries, without someone hitting it with a paintbrush every two years.

[Mark Reuten]

Any solid piece of wood has air and moisture trapped in its cellular structure. Even if it is totally vacuum packed, changes in ambient temperature are going to make that structure expand and contract thus causing wood movement to some degree.

Sealer coats are intended to reduce the porosity of the surface it is applied to so that satins can be applied without becoming blotchy and they are intended to raise and stiffen the grain so that a final fine sanding can be more effectively achieved.

You cannot stop wood movement but you can reduce it through informed material selection or lamination and compensate for it through anticipation and proper joinery choices.

In the boat building world, epoxy encapsulation is used to prevent moisture transmission in wooden structures but it only works when the wooden members are very small or laminated to reduce movement. It is a common and disastrous mistake to smear epoxy over a large timber hoping to keep water out of it. More often than not, it causes water that gets in to be trapped, causing the timber to rot prematurely.

[CessnaPilotBarry]

Sealer coats are formulated with resins that sand well with chemicals that dry fast and even out the overall porosity of the wood.. A good sanding sealer sands better than bare wood, and soaks farthest into the areas that are most porous. This makes it easy to prepare for the finish coats, and prevents the extra porous areas from drinking too much finish and doing weird things.

All that said... Lots of things can eliminate the need for a sealer coat:

- Some finishes are self-sealing, requiring no separate product. This will be stated in the product tech docs.

- Oil based pigment stains will seal the wood. These use a thin varnish to glue the pigment to the wood. This is why subsequent coats of stain often don't make the wood much darker, and is also responsible for "blotching" in woods of variable porosity. As Mark mentioned, some sealers may be applied before a stain to even out blotches.

- Oil treatments, like tung and BLO, also seal up the wood and limit penetration of film finishing products.

Sealers designed for use under water based products minimize or eliminate grain raising and eliminate the old practice of pre-raising the grain with water.

My favorite sealer under water base on woods where I desire ambering, is a 33% / 33% / 33% mix of BLO / Real PINE Turpentine (do not substitute compatible chemicals!) / Dewaxed Shellac. Grain raising is eliminated, and the color of the wood is warmed up nicely.

[RyanLincoln]

Wood sealers are made from a number of different bases with a variety of added chemicals. Some adders make the wood deck sealer perform better. In any discussion of sealers, it should be understood from the beginning that there is no one perfect product.