How to smooth wood that is splintering due to uneven hardness?

Question

I would like to sand this surface to be safe and comfy to touch:

I am using a Delta sander with grits: 60, 120, 180, 240, 320 with various speeds and applying various amounts of pressure, but it seems that I am making the problem even worse.

I don't think this answer applies to my problem, since I am not getting splinters everywhere due to low quality wood (I think), but I get entire sheets of wood that are splitting. I am using larch wood.

It seems that the sander removes material disproportionately from the softer, brighter parts of the surface. I can feel this also on the rods that connect the edge joint. There is a small mountain on each of these rods.

Answer 1

I am using a Delta sander with grits: 60, 120, 180, 240, 320

Two points in relation to this worth noting:

• you're sanding a little too finely here;
• it's worth trying sanding by hand to see if the results are better.

While many people these days do commonly sand more finely it is generally unnecessary to sand beyond approximately 180 grit. And sometimes you can get away with sanding a finished surface to 150, particularly on softwoods¹. Most finishing books will cover this, often with examples showing why it's not beneficial to sand beyond a certain grit in most cases.

With rotary sanders (both types) it is generally advised to hand-sand the surface in the direction of the grain with the same grit you stopped at on the sander. And here it might also be the case that you get better results — although possibly only slightly — sanding by hand, since you'll be sanding in mostly straight lines and because the process tends not to generate heat as power sanding does.

I don't think this answer applies to my problem, since I am not getting splinters everywhere due to low quality wood (I think), but I get entire sheets of wood that are splitting. I am using larch wood.

This is exactly the same issue, just with a different species and not to the same degree, that the other Q&A addresses.

While you most definitely can sand softwoods to a smooth finish — this is after all how most wood is smoothed these days! — how well it works is very much dependent on the wood, and as in some other areas sometimes the wood just won't cooperate. This happens all the time in planing, both by hand and by machine, e.g. when tearout occurs; while sanding tends to be more forgiving² sometimes by itself it's not enough, as here.

What you're experiencing is caused by an underlying structural issue with the wood, and neither hand planing nor scraping³ would be a sure-fire solution. With planing while the surface could be initially near-perfect, the flaking is just waiting to occur again, and can sometimes happen between the time planing is finished and the finish is applied as the surface dries slightly; additionally the actual application of the finish can cause it to recur, especially if it's wiped on. With scraping the scraper's burr can actually be prone to lifting the flakes, so you could quite easily get a worse surface than sanding is producing.

So what is the solution?
As I indicated in my Comment under the previous Question, any finish that dries hard — shellac, varnish or lacquer⁴ — can help consolidate this sort of thing and toughen up the underlying surface somewhat, allowing the flaking to be effectively taken off by light sanding afterwards. Basically you're sort of glueing the surface wood fibres together!

Commercial "sanding sealers" are sold for essentially the same purpose, and while they may have additives to aid sanding at heart they're just dilute finish (usually shellac or lacquer, because they dry so fast).

Alternatively or in addition, you can consolidate or 'bury' minor surface imperfections under the finish if you're applying a full film finish.

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¹ And if painting stopping at 120 is often acceptable!

² Hence its use as the solution to minor tearout in most workshops.

³ Note that many sources state that scraping is not a suitable technique for softwoods. This is not accurate. However, it's not always suitable and this is a case where it would likely not be.

⁴ Blended finishes such as "Danish oil" or "tung oil finish" can help as well, but they're generally not quite as effective.

Answer 2

I have to wonder what kind of a finish you are going for here. Pine, Larch, spruce etc, all have this problem, it's a fairly soft wood with denser knots and winter growth rings. This is rarely used for high quality furniture, because it is a soft wood. And on picnic tables and other things that use it on a large surface, it can still be noticeable after ware and/or the elements get to it.

Now hand sanders and belt sanders are not particularly good for making a pine board perfectly flat. Because they do what you are witnessing. Thickness planers that are nice and sharp (like the helical planers) can leave an almost perfect surface behind, and drum sanders generally will not do the uneven sanding either, since the whole board goes through with the same pressure across the whole surface.

You also have the option to try hand planes, which take a bit of practice to leave a finished surface, or wood scrapers which can create a beautiful flat top without the 'gouging' but also takes a lot of work, but doesn't need the full skill of a hand plane.

The big question is what kind of finished surface do you need and are you using the right wood for the job?

Answer 3

I've had good luck in some situations like this by using a hard and flat block to hand-sand the surface along the grain.

Specifically, I have a piece of granite (a sample from a countertop vendor) which fits self-adhesive sandpaper perfectly. This tends to keep the abrasive from "bottoming out" on the softer areas and helps to grind down the harder areas more efficiently so they both stay at the same level.

You wouldn't necessarily need a hunk of granite, but anything hard and flat should work--even a block of hardwood.

Most hand sanders have sanding pads (or plattens?) that are somewhat rubbery due to either hook-and-loop fastening systems or a cushioned surface. This allows the abrasive dig in slightly more to the sofer elements of the wood grain.

Belt sanders don't usually suffer from the problem of rubbery sanding pads, but they tend to be almost too aggressive for finish work. I think it could be done, but I've never been that deft with a belt sander.