finish penetration question

[Mark August]

When applying a polymerized tung oil finish such as Waterlox, can sanding the bare wood too finely prevent the product from penetrating the surface as thoroughly as it should? I typically apply it to wood that has been sanded to 150 grit and then sand progressively finer between coats. However, I am beginning a project where the surface has been finished with 0000 steel wool. Could this present a problem?

Mark

[Prashun Patel]

The experts might correct me, but I believe 'over sanding' is only an issue if yr trying to color wood with stain or dye or pop grain with an oil. These things require some amount of penetration. Oversanding won't hinder (nor will it help) a varnish like Waterlox from achieving a hard film.

I find that pigment stains are the most sensitive to over sanding. I find that oils do their job fine until about 400gt (for me). I have not tested oversanding's effect on dye stains.

[Scott Holmes]

Waterlox does not make a polymerized tung oil finish that I know about. Waterlox original Gloss and sealer are varnish made from Tung oil. "Polymerized tung oil" is a "marketing & sales" job on many finish labels; but not Waterlox.

Waterlox is a true, high quality phenlic resin/tung oil varnish.

Over sanding will cause problems with some finishes like
POLY, which it has adhesion problems on it's own...

Now for my next bit of advise...

Here's why I say don't use steel wool on raw wood or beween coats of finish.

1. You will leave small shads of steelwool behind. Yes you will, even if you use a magnet you will leave some behind. These will be in or under your finish... when water vapor contacts steel it rusts. No finish can stop water vapor... if it could, we would not need to account for wood movement in our solid wood construction.

2. Most steel wool has a light oil coating to keep it from rusting. Oil on the surface could cause problems with adhesion.

3. Steelwood abrades the finish and especially with poly it does not leave enough "tooth" for good adhesion.

4. And the RUST... Getting the rust out will require you to REMOVE the finish, since it's IN or UNDER the finish.

[Prashun Patel]

Scott-
Thanks for that summary on the woes of steel wool. Answers a few questions I had.

[Howard Acheson]

Mark, If you are referring to Waterlox Original finish, they are not a "polymerized tung oil". They are a standard varnish made with phenolic resin and tung oil. Once a resin and a drying oil are heated to a particular temperature, the resin and oil combine to form a new compound called "varnish".

Here is some info regarding sanding.

Sanding wood--hard or soft--beyond 220 does little more than burnish the wood making staining difficult. This is particularly true if you are using a pigment stain which sits on the surface and relies on "nooks and crannies" to impart color. Softer more porous woods can be sanded to to 220 but harder less absorbent woods may stain best if only sanded to 150. The best compromise is to aim for 180 grit.

A number of years ago a large cabinet/custom furniture shop I was involved with did series of adhesion tests with various finishes and sealers. As part of this test we explored adhesion based on sanding grit. We found about the same adhesion up to 180 - 220. Beyond 220 adhesion dropped off due to burnishing of the underlying wood particularly when non-linear machine sanders were used. This was tested on birch panels. We also found that the resulting smoothness of the first coat of finish was not materially affected by the smoothnes of the underlying wood for sandpaper grits between 150 - 220.. The smoothest surface substrate for final finishes was obtained by sanding lightly after the first coat of finish was applied and dry. Which makes the case for a thinned first coat of finish.

So our conclusion was that sanding beyond 180-220 was not necessary and could be actually detrimental.

But, most important was that there was a big appearance affect if the surface was not HAND sanded in the direction of the grain using the highest grit used on the sanding machine. A flat pad sander produced a much flatter surface than a ROS. However, both required final hand sanding with the grain for optimum appearence. If not hand sanded, swirl scratches could show. Final hand sanding using a sanding pad in the direction of the grain is a must.

To carry it one step further, sanding at 320-400 grit after the first coat and subsequent coats was the optimum. No improved appearence was noticed by between coat sanding beyond 400 for varnish. 400 was the sweet spot for thinner finishes. Between coat sanding was always done by hand whether for flattening or for adhesion.

I think you will find similar thoughts in the popular finishing books but YMMV.

Finally, the first coat of ANY finish will soak little shards of wood and cause them to raise whether the surface was sanded, planed or scraped. When the first coat of finish dries these hardened shards are what causes the surface to feel rough. Sanding with 320 paper will remove these hardened shards and subsequent coats will go on smoother. So, smoothness counts after the first coat of finish, but not much before that.

The machine finish determines the starting grit. Jointers, planers, belt sanders etc, should leave a finish that allows starting with 100 grit. From there, go to 120 grit and sand until the marks from the prior grit are gone, then move to 150 and finish at 180 grit.

Plywood is factory sanded to 180 grit. Therefore, it's best to not sand plywood except with 180 grit and sand by hand. Get the first coat of finish on and then sand with 320. That way you are sanding the finish, not the wood. This avoids sanding through today's very thin surface veneer.

[Mark August]

Thanks for the replies. I was wondering why more people don't use Waterlox as a floor finish. Most of the profesionals that I have spoken to about refinishing hardwood floors have never even heard of it. Considering how easy it is to apply compared to the stain/ poly approach, and how it produces such a natural, hand-rubbed appearance, I would think that it would be more popular among refinishers.

Mark

[Scott Holmes]

Some use Waterlox but at $90/gal for some reason people tend to buy the $30 poly...Hmmm?

[Neal Clayton]

yeah, there is that ;). i was able to get it locally last summer for 40-50 a gallon but not anymore. gone up quite a bit since then.