How to sand, stain and seal wood filler in a knothole

Question

I have this knot hole in the floor:

I want to fill it with wood filler and then sand, stain and seal it.

I have a few questions though:

1. How do I find out what sealant was used before? I assume it Polyurethane. But was it oil or water based? And if I use water based on an oil based Polyurethane, will this cause problems?

2. What's the direction of the wood fibers (wood grain or whatever it's called)? Is it in the direction of the length of the wood board?

3. How do I stain the wood after sanding down the wood filler and the area immediately around it? I mean, it's two different colors. How would that work?

4. What grain of sand paper do I need to use for the sanding?

5. Is there anything else that I need to know before starting?

6. Will the hole deteriorate if I leave it as is?

Thanks.

Answer 1

How do I find out what sealant was used before?
The answer to that is as you'd guess... look for product packaging, ask the installer or builder, etc. If it was a prefinished floor it probably has a proprietary coating that includes aluminum oxide for hardness. In any case it probably doesn't matter. I'd simply use gloss urethane for this little patch.

What's the direction of the wood fibers (or grain)?
You're correct that it's technically parallel to the length of the board. Even though there appears to be a curve in the grain direction, that's mostly a result of the type of cut made in the board.

How do I stain the wood after sanding down the wood filler and the area immediately around it?
Find a dark walnut or other stain that matches the darkest part of the knothole, and use a small art brush or cotton swab to stain just the patch. Actually, you may find a very dark wood filler that doesn't require staining.

You want to simulate a knot, not try and hide it. Also, you should avoid sanding the existing finish. I'd approach this repair from a hole-only standpoint. Do your best to avoid disturbing the finish on the adjacent area, as it will be very difficult to hide. I'd go so far as to apply masking tape to the area to preserve it.

What grain of sand paper do I need to use for the sanding?
The "grit" you'll use depends on how you apply the patch. Ideally your putty knife will leave a fairly smooth surface. Finish with 120 and 180/220 before sealing.

Is there anything else that I need to know before starting?
Just to reiterate, seek to repair only the hole, and not the surrounding finished surface.

Will the hole deteriorate if I leave it as is?
That's hard to say, not knowing the use of the floor, but it's likely that the edge of the hole will eventually degrade due to foot traffic. However, you shouldn't see serious damage unless it's susceptible to table legs, etc.

Answer 2

Any sanding and attempt at "touching up" the finish after sanding will most likely be more of an issue than the knot hole itself.

The dark ring around the knot hole may be the only weak spot, if that. The rest of the light colored floor beyond that is as durable as the rest of the floor

I would use a lacquer "burn in stick" although it take a bit of expertise, it cleans up with lacquer thinner, which should not remove any finish off the floor when used in moderation. These are available in many colors and the color you see with the stick is the color it dries to.

There are instructional videos online on how to use burn in sticks that get down to great detail but for a filler like you are looking for with no sanding, I would use a heat gun a putty knife and lacquer thinner. You can also use a log lighter or butane lighter in a pinch. Heat the stick a bit as well as the putty knife. I have pushed the hot lacquer stick into holes to get it in place. You can use the hot putty knife to do the same, it will take more time.

With the very warm but not hot putty knife slick the top of the hole off flush with the rest of the floor and use lacquer thinner to lightly dampened cloth to wipe the smear from around the hole.

As a precaution, do test a spot on your floor in a closet or someplace like that to test if the finish will hold up under the cleaning of lacquer thinner

Answer 3

You've gotten bad advice about this hole. I believe the answer you accepted the first time missed your statement that it's 3mm across, because nobody in their right mind would suggest using a Dutchman to patch a tiny hole. You also don't need a ton of sanding/refinishing for something that small, it's only two growth rings wide so the photo is clearly well-zoomed!

In your case I'd suggest a wax pencil. Not the artist variety, the wood filling ones. They're colored sticks that you smoosh in the hole then rub flat with the surface. Bring the picture with you to the hardware store so you can color match. The best bet is probably to match the color of the knot, so the patch just looks like a knot.

The burn-in stick is a viable option too. They're widely used by professional refinishers. It seems overkill here though. Color Putty is a little more user friendly, no heating and you can mix shades to color match.

By the way if that's a prefinished floor the finish is most likely some kind of conversion varnish. Take Jack's excellent advice and don't mess with it. And keep in mind that you can dig out the filler with a toothpick before it hardens if it looks wrong, sanding is a lot harder to undo.