sanding and an oil finish

[TomFromStLouis]

With diffuse porous woods like maple I find myself sanding to 400 or even 600 before applying my tung oil finish. I can see sanding marks in the wood at lower levels, so this seems quite natural and correct to me. Yet I read here and there that sanding to high grits somehow keeps oil from being absorbed into the wood. At least that is the impression I get.

1 - Does very smooth wood absorb less oil? To the extent there is less surface area, I suppose so, but if you can see those streaks that create the extra surface area clearly they are undesirable. Okay, I think I just answered that question, so let me rephrase it: how far do you go when finishing hard maple with an oil?

My questions presume that all sanding dust is removed; I use compressed air to do this. Now let's consider a wood that really polishes to a high luster on its own: persimmon. It shimmers like marble when I use my disks that claim to go to 12,000. Yet I still want some protection like a diluted wash of shellac or oil.

2 - Is there any gain in sanding persimmon past 600 when the visible lines/streaks go away? The surface itself will be the thin coat of oil, so it will have the reflectivity of the oil and not really need it in the wood itself. Would the very high polish keep any oil from absorbing into the wood? Does oil require a rougher surface to get into the wood at all? My first coats are diluted oil, my idea being that the lower viscosity helps absorption.

Any thoughts appreciated. Those based on experience especially so. I can - and probably will - run a test, but this is faster for now.

[MichaelMouse]

Burnished wood absorbs oil poorly. That's because the pores close as the wood heats. Sanding with extremely fine grain paper and pressing will heat and harden. Here's a burnished maple bowl.



Final "sanding" was with a grocery bag (paper, not plastic), and it would only tolerate a 3:1 thinner/wipe-on-poly when it came time to finish it. The heat had closed the pores just like a film finish.

You sand until you're happy. At arm's length, the science guys claim 20/20 can distinguish 300 medium-contrast lines per inch. Makes P400 or CAMI 320 fine when finishing with the grain. Film finishes may bridge/fill/diminish contrast of even larger grits. On porous woods like oak, I never bother beyond 150. Some stop at 100.

[Howard Acheson]

The issue with excessive sanding sanding with high number sand paper is that you can easily burnish the wood surface. A burnished wood surface is not as porous and can easily lead to reduced adhesion.

Here is a write-up on something we tested in my shop. It was a number of years ago but most likely is still valid.

[TomFromStLouis]

Thank you both. Howie please post the link.

[Howard Acheson]

Oops, sorry I meant to embed the write-up into my response. The write-up is from my own files--it's not stored on the web. You're welcome to cut and paste the write-up.

Here it is:

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.

[Richard D.]

Most of this I learned from Hal Taylor, I believe he has tweaked his process beyond what I have been doing but here it is.....
Indasa Rhyno Grip 100, 220, 320 followed up with Abralon 500 pad. It burnishes the wood but the advantages seem to outweigh the negatives that others have posted, at least in my experience. Deft Danish Oil wipe on and off with a paper towel, can't miss with this process but there are caveats.

After the Abralon you can see any scratches missed by previous sanding before applying oil which is a pretty big plus to me. I haven't had any adhesion problems in eight years of using this process. This process leaves a very smooth surface to touch because there is more to a finish than how it looks. It isn't the most durable so I wouldn't use it on a table top. It's simple to touch up and can always be re-coated.

I wouldn't use stain after sanding to 500 but for clear oil and varnish I have had good luck. As always, test your entire process before committing it to a project.