Finishing the cello
Finishing Approach
After much reading on the Internet I decided to go for the following approach:
- Finish the surface after raising the grain with water
- Colour the wood in a UV light box
- Deepen the colour by fuming with ammonia
- Shellac sealer/ground coats
- Coloured oil varnish coat
- Artists colour rubbed in with fingers
- 3 more coats of coloured oil varnish
- Clear oil varnish (2 coats)
- Cutting back and polishing
I found an excellent supplier of ready made varnishes and varnish ingredients in Germany at Hammerl and ordered the oil varnish and a few extra ingredients.
UV Lightbox
I then set about designing a UV light box for both giving the white cello an initial suntan and for drying the oil varnish coats.
I settled on a design that I could assemble and disassemble easily for flat storage when not in use.
The base is 1/2″ ply 610mm x 400mm into which I routed some 6mm grooves to take the hardboard sides, front and back. The sides are braced with vertical triangular cross section struts at the edges and onto which are mounting UV fluorescent fittings (4 off 18W 600mm UVA fittings on each side).
The box is 1500mm high and a top piece of similarly grooved 1/2″ ply and with a small USB fan to extract heat/varnish fumes holds it all together.
All the inside surfaces are coated with aluminium foil.
I lightly glued a piece of scrap on the neck to hold it straight and clean during tanning/varnishing. Then the cello got a final wipe down with water and a final scraping all over before going into the light box for a week or two depending on how fast it darkens up.
Here is the result after 10 days.
I then tried fuming using household ammonia (only about 9% but seems to work quite well). Here is the result after 10hrs.
Next I sealed it all with a few coats of a 1lb cut of shellac.
And then the 1st coat of coloured varnish. This is incredibly thin and only a tiny amount was enough for the whole cello.
Then back into the light box to dry.
Once dry (I left it 2 days) I rubbed down with wet 2400 grade micro-mesh and then used a mixture of burnt umber and burnt sienna artist colours rubbed in with my fingers and wiped off with a kitchen towel until the colouring was even.
Another 2 coloured coats but still looking too light so had another go smutzing with artists colours.
Next step is a final coloured coat and then 2 clear coats.
Next I cut it back with 3200 grade micro-mesh and water and then pumice and linseed oil on a felt pad. Still left some corduroy grain on the top. Finally polished to a nice matt sheen with rotten-stone and linseed oil.
Then popped off the keeper on the neck before gluing back the fingerboard.
