Filling grain and holes in burl veneers

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2 Jul 2022
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I've finally chosen the veneer combinations for my first chess board build. They're pictured here on random test blocks at various stages of experimental finishing and re-finishing. That is, they don't look their best here, but show the selection made. The walnut and maple veneers are for the dark colours (front and rear of each dark square on the board) with birch and oak covering either side of the light squares.


I'm consciously using "difficult" veneers (aka generally the most interesting looking ones) and have the expected holes, pores and grain fissures to deal with. Like these (some of which go right through)...


I want to fill these "features" individually by hand - right after initial sanding of the veneers for thickness, but before the "finish" sanding happens. There are a couple of other process steps here and it's a very convenient stage to do this. Note: this isn't a traditional chess board build, the squares remain separate individuals throughout, even when the board is finished.

I don't need exact colour matching, just a palette of very light, medium and very dark brown will do. I don't care about stain interaction either because I'm not staining anything, but I would worry about absorption of things like tung oil or the effect on hard wax oil finishes like Rubio or Osmo etc.

Sanding sealer is definitely out already and I want a wood-filler that goes proper hard too - waxy stuff won't do.

So what filler(s) would you recommend to use here?

How about wider-area filling of things like the pores in mahogany or (in this case) porous patches in walnut burl that weren't filled by glue? Could it be the same filler plastered over the surface with a putty knife, then sanded off? That would be really handy. Or would that need to be some other kind of filler?

Thanks in advance.
For the areas of dark spots mix some instant coffee in epoxy and apply using either a syringe or a sharp pointy thing

For lighter spots then sprinkle a bit of sawdust of that wood into the hole and fill with the runniest CA you can get

then when all are set and cured (a few days with this amount) sand
Thanks Droogs. I do intend to try dark epoxy for bigger holes and splits (and maybe small ones too). As an aside - some of the veneers have holes that only appear in the centers of small knots. They're quite sparsely distributed and I'm toying with the idea of filling them with resin of a bright primary colour (like fire-engine red, perhaps). For the burl I'm thinking of, they'd look like red berries scattered about on "burl bushes" (but only on a few squares, not all). One to try in an idle moment sometime.

I'm not so keen on the CA glue and dust approach as I see no way to control what the CA soaks into. It must surely block oils and finishes from penetrating once cured, mustn't it? I don't know that for a fact and would need to test it to be sure, but that "suspicion" has kept me from even trying it to date. Well, that and a perceived association with bodgery. What was the rhyme? "In glue and dust we put our trust, if that won't work then slivers must" :)
CA has been used to finish instruments, I've used it myself on a headstock.

Technique is to spread with a scrap of plastic, allow to cure, scrape back to wood, drop fill any remaining pores, scrape again etc.

I've used Tru Oil on top with success.

The gotchas are (a) fumes, outside is best and hold your breath, and (b) foaming if moisture is present.

Worth trying on scrap with pound shop tubes. If you like the effect, better quality CA can be non foaming and reduced fumes.

An alternative is Z-poxy finishing resin, same technique but slower. This yellows the wood though.
Filling of the holes has to be done before you lay the veneer otherwise you will get glue bleeding through.
My approach is to hold veneer up to a light so you can see the holes and then seal from the back face with CA glue and sawdust. Large holes may need to have patches
Once the holes are sealed you can lay the veneer and then fill any defects from the show side. Filling from the top can be with filler stick, epoxy, glue and wood dust, depending upon the finish which will be used
shellac sticks could be another option, melt some shellac into the holes, let it dry then scrape it back, repeat a couple of times until it no longer shrinks.
Thanks for your comments folks. All helpful. I've chosen and tested a process and it works pretty well. The main approach to grain and pore filling will be based on shellac - like so...

I first sand the veneers flat and smooth to get a block within 0.1 mm of its final dimensions; slightly oversize. Then into an iterated stage.

Loop: If there's no grain or pits visible then the stage is complete and the block is ready for final finishing (more shellac and/or poly with any remaining "oversizing" ground off just prior to that). Exit loop.

Otherwise, apply several heavy spit coats of shellac, let 'em dry, then rub it out with 400 and 600 grit paper (stuck on hard, flat blocks to keep surfaces planar). This rub out is *very* aggressive and takes most of the surface back to bare wood and maybe even a bit beyond (subject to staying within size constraints which are continually checked with a dial gauge throughout). Go back to Loop:

Considered in isolation, this is almost certainly *not* the ideal solution, but in the context of the existing construction phases for these blocks, it slots in seamlessly. The issue is that the blocks must all be *precisely* 65mm square and 26.9mm thick - veneer thicknesses included! So I can't just keep on sanding until a surface is blemish-free as a block could get too small (or I could even sand through a veneer). I have to sand each veneer down by a fixed (and maximum) amount while keeping faces square, parallel and flat. Building up the surface with shellac before rubbing it back down again makes that relatively "risk free". Blending this process with "final dimensioning" doesn't alter much (process-wise) and feels perfectly natural in practice.

The one thing this doesn't deal with is the really big holes that let too much glue through. Small dots here and there, or a sheen of glue over a porous section are tolerable, but the big blobs are unsightly. I'm going to adopt @Hornbeam ('s) suggestion and fill those bigger holes prior to laying the veneers.

Here's a few pics of the blocks I tested with - should help illustrate points made above. Note: none of these veneers/blocks are "finished" in any way - just rubbed back to 600 grit.

A piece of oak burl after one iteration of spit-coats and rubbing out. Lit to show only sheen, and the surface flaws. This is about the worst I'll have to deal with (I hope). Looks awful in a photo - didn't look that bad "in person".

A different piece of the same oak veneer after three iterations of spit/rubout. Lit the same. Pretty much done.

Same completed veneer re-oriented to show sheen and surface detail.

And finally, just the surface detail.
burl oak rubbed out 600.jpg

Here's the other side of the same block that shows an ugly white glue blob (in the darkest bit). I'll fill those before veneering now.
kirlian birch rubbed out 600.jpg
Super glue is best applied with foam sponge , Just finding the right kind. Some kinds set off straight away others give you a little working time. Watch out for the fumes they can be really aggressive (not sure if they are dangerous but they really get in your eyes and lungs)
On walnut burl veneer, after sanding etc, I spray clear base coat, then use base coat in a syringe to go and fill all the holes and cracks.

Scrape off with a card scraper or sharp blade, sand again and topcoat.

I've seen some people use CA glue too.
I've frequently finished small box burl veneers using nothing more than building up coats of Liberon fine finishing oil and sanding back after each coat until the grain pores are filled. It requires anywhere between 7 and 10 coats but leaves a superb finish. Left a day between coats and a week after final coat before using a nylon scotch brite pad to gently denib any dust particles or inconsistencies and to take the gloss back to a satin sheen, using some carnauba and beeswax on the pad. Then a further two coats of wax (a day between coats) for a fine, deep lustrous finish which shows the grain in all its glory to its best. For showcasing such lovely grain I would only ever use oil based finishes.