What is filler? A binder, like polyvinyl acetate or acrylic suspension or hide glue or linseed oil, and a filler like chalk or plaster of Paris or marble dust.
I've been experimenting with gesso and/or filler types today. I want something a little more substantial than acrylic gesso to smooth out wood grain, as well as bigger things like knots or holes. I've used various wood fillers to fill and repair or restore defects in frames but none are quite perfect, all a bit different.
I decided to use chalk (whiting, calcium carbonate) rather than mixing things with an off-the-shelf filler. I have plaster in stock, and even some super hard modern resin plaster, but I don't want a surface so hard that I can't sand it. The best fillers tend to be rather flaky and very easy to sand, but sticky enough to hold together. Here are some first results:
I started with a mix of chalk and water and PVA. The mixes were very watery at first (far right) and I wasn't impressed, but now its dry it does look rather smooth. Next to that I simply used less: 1 chalk to 1 PVA mix (which was 1:3 PVA:water). This was a little more syrupy and has held a few peaks, but still to watery to fill deep holes. One advantage of PVA is that it is sticky, and can be wetted to apply other layers (gilding). Water resistance is all well and good, but often things work better is a sort of symbiotic state of absorption and release, so I think that some water sensitivity can be an advantage to stability.
I then tried a mix of chalk and gesso itself, Golden Sandable Hard Gesso. This made a rather nice paste which was gluey and pleasant to work with. Not so liquid for a grain filling paint but good for filling larger gaps, I surmise. I'll see how it sands out. Then I tried mixes of chalk and basic acrylic paint; here a 1:1 mix of Golden Fluid Carbon Black and water, which is the supreme black paint, the best black of any I think.
A 1:1 paint and chalk mix (far left) was again very watery, so watery it was more like water than paint. I tried a 3:1 chalk:paint mix next and this made a syrupy mix, more fluid than honey, but still with some thickness. This seemed like a great choice as a general grain concealer. I used this last mix on some raw wood to dry as a bigger test.
Only the Golden Gesso and chalk mix covered the grain completely with great ease - I suppose this depends entirely on how it is applied. I used a spatula, then a sponge as a test but the sponge application was so thin (though, of course, very homogenous) that it didn't fill or conceal grain. I deliberately chose rough, unsanded and unplaned wood. The 'dots' all over the sample above are bits of this wood, not chalk particles.
In effect I'm making my own acrylic gesso. I need more of a clearly defined use to measure actual success. A primer layer for bare wood is one, and I want a secondary, more gap-filling solid filler, to cover knots, holes etc. which would be applied first. Most general fillers are too coarse for my use; the best I my favoured one is Wilkinson Fine Surface Filler, but perhaps this isn't liquid enough for a top coat.
One option is something solvent based like Paraloid resin with chalk. I may try this as a filler, as water and wood always reacts a little badly. Anything oil based or non-watery tends to work better with wood.
