Grain Filling with Shellac

[Harvey Pascoe]

Since this forum has relatively few topics I decided to start a new thread as a continuation of a previous.

I think it was Scott Holmes who suggested to try grain filling with shellac. Sounded improbable to me, but last night I did some experimentation. After thickening up my jar of shellac by adding more flake - at this point I have no idea what the ratio is - but I'd guess around 1:3 or 4. Then put 3 coats on India rosewood, a moderately fine grained wood, sanding between coats. At least from what I can see under magnification, it seems to be working well, but only time will tell after applying the first coat of varnish.

Several things occurred to me about grain filling and one is that capillary effect plays a major role. Capillary effect, along with osmosis, is the mechanism that makes the tree grow. Capillaries conduct fluid along its length via a method known as aqueous tension. However, when the cell is exposed by cutting the wood, the effect is altered so that capillaries, particularly short ones, can also eject the fluid we attempt to fill them with. This becomes very apparent when we apply varnish or acrylic to very porous wood like mahogany or walnut. The finish is ejected from the exposed cell and mounds up along the upper edges of the cell, so that if you don't sand between coats, the finish just keeps mounding up and the cell doesn't get filled. Think of this like a ditch digger who throws the dirt up along the margins of his ditch.

Therefore, when we finally end up filling the cells with finish, what we are really doing is not filling, but bridging the upper opening of the cells until they are closed, unless we find a way to prevent the cells from ejecting the fluid.We see what happens when we attempt to force a filler into the open cell as it often pops back up out of the cell because the filler is trying to compress the air in the cell. Note that all of this is highly dependent on cell size and viscosity of the finish or grain filler. Now cometh shellac.

The viscosity of shellac due to alcohol is extremely low and because of that it appears to me that the cell does not eject the shellac by capillary effect and therefore the cell appears to fill up with solids somewhat faster. Further, there is no bridging effect and the filling is COMPLETE and will greatly reduce the tendency for the finish to shrink over time , sink into the unfilled cell, and result in finish checking.

I think this explains why I've had so little success with water base fillers ( and others report the same). I'm not suggesting that thickened shellac will work well on grain porous woods like oak or ash but it may well be worth a try on the likes of walnut. Next up I will try cocobolo which is highly variable as to cell size, often being quite large and deep.

Does any of this make sense to you?

[Phil Thien]

Does any of this make sense to you?

Yep. I'd be interested in hearing whether this would work for oak. Would it be possible, when you're trying it on the cocobolo, to also try a small piece of oak?

[Harvey Pascoe]

Yep. I'd be interested in hearing whether this would work for oak. Would it be possible, when you're trying it on the cocobolo, to also try a small piece of oak?

I don't have any oak but I can try it with some ash.

[John TenEyck]

Most woods that are open pored will accept filler. Perhaps you haven't used an appropriate filler or technique if you haven't had success in the past. Mahogany, walnut, oak, etc. all fill pretty easily. Fine pored woods are a different story and I can see how shellac could work better with them. Surface tension, more than viscosity, might also play a role in whether or not one material wants to fill the pore or coalesce around the rim.

[Prashun Patel]

You're probably more of a scientist than I, but I've been able to fill grain with shellac, several kinds of varnish, and even oil/varnish/woodslurry.

I always assumed the reason shellac does not 'bleed back' is bkz it dries so darn quickly. Oil varnishes tend to bleed out of most woods because they remain wet long enough for the cells to spit. But shellac is like Medusa.

I did not think grain fill was a function of capilary action in the cells, but just a function of the craters in the surface of porous woods. You can really fill those craters with anything that's fine enough to fit inside them, even drywall compound or glue will 'work'.

Shellac will even work to completely fill the grain on highly porous woods like oak and ash. It just may take a while.

[Howard Acheson]

Here is an excellent article on the same subject by a very well respected woodworker/restorer.

http://www.hardwoodlumberandmore.com...h-Shellac.aspx

[Harvey Pascoe]

Most woods that are open pored will accept filler. Perhaps you haven't used an appropriate filler or technique if you haven't had success in the past. Mahogany, walnut, oak, etc. all fill pretty easily. Fine pored woods are a different story and I can see how shellac could work better with them. Surface tension, more than viscosity, might also play a role in whether or not one material wants to fill the pore or coalesce around the rim.

Sure, pigmented filler stains work just fine but we were talking about gain filling with transparent fillers. I've sanded varnish and oil into the wood. Yuck, I didn't like the result.

[John TenEyck]

Sure, pigmented filler stains work just fine but we were talking about gain filling with transparent fillers. I've sanded varnish and oil into the wood. Yuck, I didn't like the result.

In addition to pigmented fillers, you can buy transparent paste fillers - I think they are silica based. In any case, they do exist, and are an alternative to the shellac method.

[Harvey Pascoe]

I finished up the India rose wood grain filling today, 4 apps of 1:3 shellac/alcohol and then gave it a shot of varnish. Near perfect. I also tried some quarter sawn ash, also 4 apps of same. Not completely filled but looks pretty good with one shot of varnish. Really needs two more shellac to do the job. Yeah, Phil, it works fine.

Filling and sanding is fast and easy, none of that malarkey of using a squeege across the grain and making a big mess. Plus, if you want to use it for color change, you can do that too. Going darker, not lighter. I did this on unassembled parts for a box which was nice in that it prevented the usual problem of unseen glue spotting.

How long did the filling take? Did it in stages while doing other things but I'd say not more than twenty minutes total, excluding mixing and clean up.

[Scott Holmes]

The author of this article is the guy I call when I run into trouble. Steve is a friend and a FINISHING GURU.

[Harvey Pascoe]

Just a side note here, going 1:3 on the shellac ratio poses a much longer delay in drying unless you have excellent drying conditions like putting the work out in hot sun, so I kicked it back to 1:4 and that brought dry time down to about 15 minutes. In the shop w/o sunlight, it was more like an hour, which defeats the purpose of speed.

[Howard Acheson]

Just a side note here, going 1:3 on the shellac ratio poses a much longer delay in drying unless you have excellent drying conditions like putting the work out in hot sun, so I kicked it back to 1:4 and that brought dry time down to about 15 minutes. In the shop w/o sunlight, it was more like an hour, which defeats the purpose of speed.

Putting it out in the sun probably had little to do with the speed of "drying". Shellac dries as the alcohol evaporates. Temperature or sunlight has little to do with it. But, putting it out doors probably exposed it to some sort of breeze. Air movement will accelerate the evaporation process. But, so would placing a fan to create air movement over the panel.

[Harvey Pascoe]

Yeah, there was a good breeze but I also have ceiling fans in the shop and that didn't seem to help much. All I can say is that the sun appeared to speed it up, but there is also the risk of it getting too hot and blistering, that happens occasionally when I'm not paying attention. The solvent then gassifies too fast and does not escape thru the outer film of dried finish of any kind.

[Prashun Patel]

Sunlight has nothing to do with evaporation. Sun heat does. So putting it outside in the hot sun will certainly speed the evaporation. However, it will (and I say this from modest experience) heat the surface most, which will cause premature drying of the top layer, which can impede the underlying finish from drying efficiently. This may cause subsequent coats to remain soft or the overall surface to remain soft.

Even shellac may appear to be dry, but should really be given good time to dry thru and thru. When I'm applying shellac, I only do several coats within the hour if they are very thin cuts. But if yr schedule works for you, power to you!

[Anne Schmidt]

Thanks all . . . timely subject as I am NOW trying to fill oak with water-based paste filler(famowood) and not liking these dark spots or the length of time it took to pigment it to the color I wanted, esp. given how it changes colors both after it drys and after it's coated!!! (shellac method makes color matching unnecessary.

So, I may try this shellac method(I love shellac).

Note on surface tension: alcohol does seem to relax surface tension; I have noticed that when just a couple of drops of alcohol are added to a water-clogged ear, the whole shebang---water plus alcohol drops right out. Thanks for bringing this concept into the wood finsihing world.

Surface tension, more than viscosity, might also play a role in whether or not one material wants to fill the pore or coalesce around the rim

Anne Schmidt