Quote:
Originally Posted by Tim McKnight
Hi Ed. That's a great question. My first coat of filler will fill 90% of the open wood pores. I sand the first filler coat level to the surface of the wood using 220 grit and this sanding course does expose some bare wood. The second and final [skim] coat of filler covers any bare wood which was exposed and generally fills the remainder of any open pores in the wood or any new pores that were exposed during sanding. The final filler sanding course, with 320 grit, is basically to mechanically abrade the filler to provide a tooth for the sealer to adhere to with a goal of NOT exposing any bare wood.
The trick to the entire filling process is to NOT apply the filler too thick and to judiciously squeegee ALL excess filler from the surface of the wood. As you eluded to in your question, excess filler build up on the surface of the wood would indeed have a negative impact on the wood's ability to vibrate.
I use a 4" wide auto body "bondo" spreader to apply the filler and squeegee most of the excess off with. As a follow up I use a 12" wide rubber window squeegee to do a final removal of any ridges of filler that may be left over from the edges of the 4" spreader application course.
I apply a LOT of downward force on the application spreader to force the filler into the wood pores. The downward force bends the spreader at a 45* angle to the surface of the wood and hardly leaves any filler standing proud of the surface of the wood. This process isn't like icing a cake to leave a thick layer of icing (filler) on the surface. This is more akin to scraping ice off your car's windows so you can see through the glass.
I hope that makes sense?
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Tim,
I have always wondered about the grit of sandpaper being used on tonewood. To my way of thinking,320 grit paper is extremely rough and would leave deep scratches, which would then require finer grits to sand out the scratches.
My reference points come from my career in the automotive industry. Various components that have been heat treated for example must be hardness tested to ensure they meet the required specifications. Now, I have considerable experience in polishing small test parts that will be placed under a Microvickers machine for the purposes of determining this surface hardness. These surfaces must be polished to a mirror finish and completely scratch free. I usually start with a 320 grit paper but work my way up to 1200 grit paper and finally a buffing pad to achieve the needed results.
In working with tonewoods, why are you not striving for a completely smooth surface finish before applying the lacquer? Or is 320 grit smooth enough because the wood surface will eventually be covered up? Or maybe it’s because you aren’t polishing the wood surface, but rather leveling it in preparation for lacquer? Sorry for so many questions, but I’ve wondered about this for some time.
Love your threads BTW, especially the engineering that goes into what you do. We’re I ever able to have a guitar built, I can’t imagine having anyone but you build it.