Can I use a hardening wood filler over softer wood putty on a walnut island?

Question

I have a large walnut island in my kitchen. It is not end grain and we do not cut directly on it. It was made and installed 12 years ago. The few knots holes and cracks were filled with either wood putty or melted wax, either way, it never hardened. The initial 'finish' coat was a beeswax cutting board paste.

However, the paste didn't stop water spots from forming and didn't offer great protection from normal wear. I did lots of research and ended up using Rubio Monocoat. It has low VOCs, is very easy to apply and has worked beautifully. It does wear off over time and I have reapplied it once and it needs to be done again.

It has always bothered me that the filler was putty, and soft. I know the craftsman did this because it was easiest to fill these oddly shaped areas with a dark putty. However, the areas are now sunken in so I would like to make matching filler. Since I have a few small surface scratches, I want to be slightly more aggressive in sanding prior to applying the Monocoat this time.

Which brings me to my question regarding the knot holes and cracks. I usually lightly sand the island, by hand, but plan on carefully using a random orbital sander this time. This will also result in producing sanding dust that I can easily use. Can I use a shellac/wood dust filler over the putty, or should I try to remove the putty before filling?

Can you use a hard filler over a soft one? Yes, but I wouldn't. Sorry don't have time for a formal Answer but will try to add one later this evening if nobody else posts one in the meantime. In short though, I would recommend digging out the old and filling from scratch it as you suggest yourself (although not with shellac + sanding dust). — Graphus, Oct 18 '21 at 17:56
Thank you. I can attempt to dig out the putty filler, I doubt I can get it totally clean. I would assume (althought I probably shouldn't) that you would recommend digging it out so that there will be a firmer base for the new fill that will harden, correct? I shouldn't have to worry too much about compalibility issues betweent the older putty and the newer fill, should I? I have used wood glue and saw dust as a filler that blends very nicely on the finished product. Would you recommend glue and saw dust versus shellac? I thought shellac may work better for this application. — teri, Oct 18 '21 at 18:02

gnicko · Answer 1 · 2021-10-20T00:19:14.783

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I would consider removing the current filler/putty and filling the knots with a colored epoxy. There are tints, described by their distributors as being "food-safe" which are sold specifically to color epoxy for use in countertops, bar tops, etc. although there are many ways to successfully tint or color epoxy.

Regardless of the coloring agent, a dark-brown/black color would blend nicely with the walnut and be almost unnoticeable.

Once the epoxy cures, sand and wax/oil as usual.

edited Oct 20 '21 at 00:19

answered Oct 18 '21 at 18:40

gnicko

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The food-safe tint thing is just a red herring with this kind of thing Greg. There's no evidence that even a toxic pigment (and there are few enough of those left) would pose a risk to the user in this context, same as the heavy-metal driers in varnishes aren't a risk based on all available evidence. – Graphus Oct 18 '21 at 22:09
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@Graphus Yeah...but people get all worked up about it so, several brands of tints are marketing themselves as "food-safe" (as are finishes, etc.) and if I didn't include the buzzwords, somebody would be climbing all up in my shit for proposing unsafe actions or some such.... – gnicko Oct 19 '21 at 13:53
I know what you're getting as but to be fair, I've never gotten any flak for suggesting using oil or enamel paint to tint epoxy. And we should, as much as possible, be doing our part not to perpetuate myths like "there are food-safe finishes and non-food-safe finishes" cos, there ain't ^_^ – Graphus Oct 19 '21 at 17:53
I don't want to drag this too far off topic, but it really boils down to what you're talking about when you say "food safe." There's a difference between safe for casual contact, safe for prolonged contact, safe for contact with hot foods, safe for cutting... I'd eat an apple sitting in a bowl finished with BLO; applesauce or soup from the same bowl, not so much. The unqualified notion that once cured, all finishes are food safe is bogus; there are plenty of finishes that nobody would call "food safe," and there's no doubt that some finishes are safer than others. – Caleb Oct 19 '21 at 18:22
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@Caleb question: have you ever heard or read of anyone, child or adult, being poisoned from contact with a cured, non-pigmented finish? – Volfram K Oct 20 '21 at 05:45
@VolframK No, but that's not the point. Would you sell someone a product meant for food contact knowing that it could contribute to elevated levels of heavy metals in their blood? No finish is indestructible; they all wear off, and they don't just disappear. – Caleb Oct 20 '21 at 07:04
@Caleb, you say you don't want to drag this too far off topic, then attempt to introduce the broader topic, which is off-topic; and using an obvious straw man :-| The context here is very clear, of limited scope, and there's nothing whatever to concern the OP. – Graphus Oct 20 '21 at 08:24

score 2 · Answer 2 · answered Oct 18 '21 at 22:05

Can I use a hardening wood filler over softer wood putty on a walnut island?

It would be much better to remove a soft filler and fill any holes from the bottom up with a hard filler, otherwise you risk the hard filler being undermined by the soft material underneath — sort of like building a house on loose foundations.

It might just be possible for a hard filler 'cap' to work its way loose in time, after a few cycles of expansion and contraction. I have certainly had shallow fills that didn't bond well to the depression they were in¹ be dislodged during subsequent sanding.

From the Comments:

I doubt I can get it totally clean

It's not absolutely necessary to get every trace of the original filler out of the holes (and practically, this can be virtually impossible) but do aim for mostly clean.

If the previous filler was any of the usual kinds of wood putties they tend to have a fairly dry and crumbly consistency, so it may not be difficult to remove most of it once you get at it with a pick, awl or other sharp tool!

Once the bulk of it is gone brush out as much of the debris as you can using stiff brushes, even some fine wire brushes if available (brass or stainless steel are reasonably easy to get these days and very cheap).

And if necessary wash the interior with a solvent of some kind if you want to take the cleaning one step further.

If the previous filler was a wax cleaning with a solvent would be of particular importance since wax is a 'universal resist' and basically nothing will stick to it (other than more wax). You can use mineral spirits, VM&P naphtha or paint thinner if you have any of the three. Don't skimp on the solvent and use plenty of paper towels so you aren't just spreading around the dissolved wax but are actually soaking it up and removing it.

Would you recommend glue and saw dust versus shellac? I thought shellac may work better for this application.

Glue and wood dust would be preferable to shellac and wood dust for anything but the shallowest defects. In addition to shellac filler material of this kind² taking longer to dry for a deeper hole (because the alcohol has to work its way out of the interior before it can evaporate) it will just plain be weaker.

If you go with the DIY filler you're familiar with based on a PVA wood glue it can be coloured a tad darker if you wish with a dab of acrylic paint. Even though the glue & dust fill will naturally be darker anyway (approximating the colour of end grain once the finish goes on) defects in wood do tend to be quite dark, often nearly black, so fills can look best if a good bit darker than the surrounding wood.

Epoxy as the binder material is also an option but a little more difficult to get flush with the surface because it's so hard once cured.

¹ Either because I didn't prep the surface properly or didn't provide any undercuts for the filler to grip.

² Commercial shellac fillers (AKA burn-in sticks) are a different thing entirely, very hard and tough, because there's no solvent in them.

Thank you so much for the information!! I hope to tackle the project tomorrow and I certainly feel more prepared after your input!! I really appreciate all of y'all's time (yes, I am southern! ;-) ) — teri, Oct 20 '21 at 00:14

Can I use a hardening wood filler over softer wood putty on a walnut island?

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