This is a step I try to take as early in the process as possible. Almost all fillers shrink, so the more the filler shrinks prior to the top coats of varnish the better. When I worked for Stefan Sobell, all the lacquering was done by Dave Wilson. I’m not sure what process Dave uses now, but at the time Dave used lacquer itself as the filler. But used a lot of it to do so. As he told me years ago “There is more lacquer goes on a guitar than on a car.”
Ultimately, the grain needs to be filled with something.
When I was in the UK and in Germany I used a spray shop in the south of England called Bow Finishing. The lads there do a truly magnificent job, they spray nitro, PU or Polyester finishes. They sprayed the Session King guitars with Polyester, which is (as they told me) “basically, sprayed epoxy.”
Epoxy fills are now standard for many guitar makers. But there are hundreds of epoxies. They are not all the same. Many yellow with age, many crack with age – they’re too hard and inflexible. Others are too soft for use on a musical instrument. You have to pick the right one.
I’ve followed the lead of other makers in Australia and use one made for the boat building industry, specifically the Australia boat building world, where the conditions are that bit harsher than say the UK.
But prior to the grain filling is another step – I give the instrument a wash coat of shellac. Most makers go for a dewaxed, bleached shellac (if they use shellac at all) but whilst dewaxed , bleached shellac sounds good in theory, its the least durable of all the shellacs. This was something I found out from the shellac producers themselves on a trip to India back in 2013. I use shellac which has a natural dark tint to it, and really turns up the contrast when you use it on a wood like mahogany, and adds a beautiful glow to torrefied sitka.
Then its time to fill the grain and build a finish.
You have to be careful with epoxy finishes. Its easy to put it on, and to leave it on, too thick. On the back and sides, this isn’t so much of an issue. Actually, it looks great. But on a soundboard, a thick finish can be a real tone killer. Mind, if you don’t have enough lacquer on a soundboard you do nothing to protect it. And whatever you put on, it will shrink with time. You may feel proud of your “ultra thin finish” but you might not be when you see the guitar again in 6 months.
As luthier/teacher Robbie O’Brien says – “The thing about finishing isn’t how much you put on, its about how much you leave on.”
He’s right. You have to put enough on, so when you sand the finish back to flatten it, you don’t go through and back to bare wood.
So. the epoxy is mixed, and thinned and pushed into the wood pores to seal it. At this stage, many makers scrape the epoxy off, or sand it back to the bare wood, then apply the top coats. But in order to get a nice flat surface a lot of lacquer needs to be applied and then leveled. Its a toss up – do you want a thin flat finish? Either way, you have to apply a lot of something in order to level the surface prior to polishing. Finishes like nitro-cellulose are “low-solid” which means you have to put a lot on to build up a layer thick enough to level. Epoxies are the opposite – they are “high solid.” You have to put it on sparingly in order not to leave yourself a huge amount of sanding to not kill the sound.
With the Session King instrument I make, the epoxy is applied in several coats with a foam roller, left to dry, then sanded back to a flat surface. This is repeated until the grain is filled and the body has a nice even flat film of finish covering the whole surface. This becomes the perfect surface to spray the top coats. In the case of a Session King, the top coats are satin nitro lacquer. For my more expensive work, the top coats are high gloss nitro lacquer which are leveled and polished on a buffer to a high gloss.
Now at this stage I’ve not even fretted the neck, but its worth stopping the woodwork to final sand, then seal the body. The sealer really helps protect the body. At this stage the finish is much thicker than it will be at the end, so really helps protect the instrument whilst I’m working on it.
Next step is to reattach the neck, and tap the frets in.