How smooth and finish adherence

[David Ragan]

If I did finish to 320 or 400, or whatever.....has anyone had a finish that didn't adhere? To me, this sounds impossible-for the finish to not adhere.

To take it further, I have finished a jewelry box with the abrasive cloths that are used to buff out aircraft windshields (MicroMesh for wood), and the surface felt so smooth I did not apply a finish. Has anyone ever used Micromesh and put a finish over that?

Hi, I posted the above starting in the General Forum, but now probably belongs here. What started me on this was a Paul Sellars YouTube about how its really not necessary to sharpen/hone beyond 250 your plane irons. He said he would in fact have to roughen up the wood for the finish to adhere. While he is obviously way above my level, I am not sure about this. You know, all the sanding and fine honing that we do with all the 12K waterstones and above being counterproductive.

Is it?

[Dave Anderson NH]

There is no correct answer David. Even as a hand tool woodworker I will often take a finely hand planed surface and sand it lightly if I am dyeing the wood. This allows more even absorption of the dye on non figured woods. For highly figured woods you intend to dye things are different since uneven absorption helps to highlight the figure. While I have never looked at a Paul Sellers video I suspect you are missing or misunderstanding something he is saying. I can't conceive of sharpening a plane iron to such a coarse level. I sharpen my smoothers to as high and fine a level as I can get them as a means of both getting a good surface and to help prevent tearout. While I primarily use shellac as a finish on furniture since I do mostly period work, I do use oil based varnishes occasionally and once or twice I've used water based. None of them have presented any problem with getting a finish to adhere well to the wood.

[Howard Acheson]

Here is something that be useful.

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 finish left by the sizing machine 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.

[David Ragan]

Dave & Howard.....those are great answers. Also over in neanderthal forum
so what you mean by finish failure is that it flakes off or contracts/wrinkles? Probably a variety of presentations of failure.
There are so very many factors involved-
i will have to dig out all this excellent collective wisdom (sincerely) before committing my next finish