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?
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