Filling those tiny cracks in hardwoods

[Bob Crimmins]

I'm finishing flat hardwood boards and I'd like the finished surface to perfectly smooth. I'm starting the project with walnut and trying to find the best way to fill those tiny, hairline grain cracks. Should I just use colored wood filling crayons? Is there a filler that's best? I've tried flooding them with CA glue. That seems to work, but I'm worried about whether the CA will react well with the final finishing substance (e.g., block oil, Rubio Monocoat, Osmo Polyex, etc.)

Is there a best practice for this sort of filling?

[Stan Calow]

There is CA glue that comes already tinted in black or brown (Starbond is the brand I use). I like to tape off the area first to avoid the glue spreading over the surface. If it will fit, I will break off pieces of veneer (of the appropriate wood), coat both sides with CA and slip it into the crack. Sand or cut flush as necessary.

[John K Jordan]

I'm finishing flat hardwood boards and I'd like the finished surface to perfectly smooth. I'm starting the project with walnut and trying to find the best way to fill those tiny, hairline grain cracks. Should I just use colored wood filling crayons? Is there a filler that's best? I've tried flooding them with CA glue. That seems to work, but I'm worried about whether the CA will react well with the final finishing substance (e.g., block oil, Rubio Monocoat, Osmo Polyex, etc.)

Is there a best practice for this sort of filling?

For a tiny crack, tearout chip, or other void I sometimes use a method I learned from John Lucas: wet sand with CA glue. This avoids the problem of the CA discoloring/affecting the finish where applied and fills the void with fine sanding dust from the immediate area so the repair is invisible. The method is to apply a bit of fine CA to the sandpaper then sand the area by hand. Repeat if needed. The glue might stick the sandpaper to the fingers but it's not life threatening. I pick the grit based on the wood and the defect, often 400 grit, occasionally slightly coarser.

Of course if the hairline crack is due to the the wood shrinking because it's not completely dry or seasonal movement, nothing may fix it. Any fill might simply be temporary if the crack is getting wider.

JKJ

[Lawrence Duckworth]

For a tiny crack, tearout chip, or other void I sometimes use a method I learned from John Lucas: wet sand with CA glue. This avoids the problem of the CA discoloring/affecting the finish where applied and fills the void with fine sanding dust from the immediate area so the repair is invisible. The method is to apply a bit of fine CA to the sandpaper then sand the area by hand. Repeat if needed. The glue might stick the sandpaper to the fingers but it's not life threatening. I pick the grit based on the wood and the defect, often 400 grit, occasionally slightly coarser.



JKJ

slick idea!

[glenn bradley]

I'm not clear on what sort of defect we are talking about. Are these actual cracks in the material? Are you talking about doing something like a grain filler? A picture of your surface would get you better answers.

[Scott Holmes]

Brush on a coat of de-waxed shellac... sand it back to flat; repeat; repeat, until you get a uniform dull surface after sanding. Small shiny spots show you where you haven't completely filled the surface. Shellac is colorfast and will provide an excellent surface for your final topcoat.

[Don Parker]

Another way of doing this is to fill the grain and any other imperfections with a few separate treatments of System III Silver Tip epoxy, sanding flat in between treatments. You mix it, then spread it with a foam brush, or use a plastic card or squeegee to spread it around. The goal is to keep the coat thin, but to force it down into the grain and other imperfections. Let the first treatment harden overnight, sand flat, then repeat. Eventually, you get a glass smooth surface, which is also a great sealer coat for many finishes to go on top. You can sand back to wood, but you run the risk of opening pores or other imperfections that are hiding right under the surface. Leaving a thin coat of epoxy on as the first coat of finish solves that problem. Not all epoxies are good for this. Silver Tip is ideal. Zpoxy works, but you have to clean the surface after it hardens to prevent top coat adhesion problems caused by amine blush. No such problem with Silver Tip.