After reading this and other posts, and doing quite a bit of additional reading, I decided to refinish my 1977 Mathias Thoma which was extremely scratched up and had graffiti scratched into it. Since I own a wood shop and the bass was very cheap and beat up I figure I had nothing to lose. I did use a random-orbit sander (very carefully) with 150-grit discs on the large areas and hand-sanded and used a set of cabinet scrapers on the details. Then hand-sanded it all to 220, then 400 grit smoothness. It took about 8 or 10 hours of sanding to completely strip the bass. The old spirit varnish was very powdery and came off easily, barely loading the sandpaper. I put a lot of time into cleaning up the joints and details with the scrapers.
On the advice of a luthier friend I used egg yolk for a ground - adding (on my own judgement) some denatured alcohol and shellac. Well the yolks curdled and left quite a bit of residue as well as raising the grain. I'd ask more questions before I do that again! Sanded all the mess off to smooth again.
Then applied 5 coats (so far) of Tru Oil tinted with artists' oil colors. It dries beautifully in just an hour or two to a medium-high gloss. After the second coat my luck ran out and it was almost impossible to apply without streaking as the Tru-oil really does not flow out at all. Maybe it needs to be thinned, but the coats are so thin already! It's hard to build the color - after the 5 coats it's still an unattractive peachy tan. Using very short strokes and a fine-weave cloth to apply the varnish minimizes the streaking. I went and looked at a bunch of expensive basses the following week and they were a lot more streaked than mine, so maybe streaks in the varnish just really aren't considered a defect?
On the other hand some new basses with spirit varnish look like they have been dipped in it, with no shading or color variation at all. I would really like to know more about applying the varnish to get it darker.
I had to go play the bass, so I let it cure for a couple days then strung it up. I was quite pleasantly surprised at the improvement in the tone!!! It is much louder and clearer with the Tru-Oil finish.
I figure I will add some more coats to darken it. The figures in the grain look like they are swimming in a 3D clear amber pool. The varnish has a lot of depth (in the good spots).
I had some trouble with dust settling on the bass even with the quick dry time of Tru-Oil. I sanded to 400 between coats and it took off the specks (just the way I handled it with my limited experience). Also some dried, tiny flecks of varnish fell back into it from the edge of the container and I had to pick a lot of those out of the finish.
I would not recommend using Watco on a bass. I have used it a lot on my woodworking stuff and although it's better than many oil stains, it's still a stain not a varnish. It's gummy and it highlights the tiniest scratches. There is a long dry time and it requires very judicious sanding to smooth the grain without loading the sandpaper or taking off too much stain. I'm not crazy about the color choices either (I have used it only on birch though). Just my $.02.
Before and after pics of the bass attached!
