I have been building several guitars (for grandchildren), but have encountered a problem with the tops developing a washboard effect. The tops are all at least 20 years old. They had a wash coat of 5 lb cut shellac diluted 6 to 1 with alcohol. That was followed by a coat of LMI oil varnish 1 part brown to 5 parts amber (to get that old Martin color).
True confession time: The LMI colored varnishes were a couple years old, and the color turned out very blotchy. I removed the varnish and shellac with Soy Gel paint stripper, washed the tops and then re-sanded, wash coat, colored varnish, (new), and then 3 coats of Pratt & Lambert clear gloss varnish with light sanding between coats. All of the tops have the same wash boarding to varying degrees.
In the past I have used the LMI colored varnish followed by LMI clear oil varnish, and didn't have this problem. Used the same brush.
Reading over this, I think I may have answered my own question. I wonder if using the paint stripper somehow allowed the softer wider growth rings to absorb more water. The sanding then took away more of the swollen, softer part, and when dry it left an indentation.
I would certainly love to hear any other ideas about this problem. Any suggestions as to remedies? I'm reluctant to go with the Soy Gel stripper, again, but sanding away the varnish and indentations sounds pretty labor intensive.
Thanks for your help, George
Correction. It is the narrower, darker growth rings that are indented. Is the above explanation still possible?
it looks like cracks in the wood. If you are using any thing water, wate a couple of days to let it dry.
Your stripper might have water in it. It could need a grain filler first then the finish.
Perhaps. after the stripper and the washing, it didn't dry long enough. Heck, I'd leave it a week suitably supported against warping. After that, you could try ironing it using a cloth saturated with water, as you would when trying to remove dents. At this point you could try grain filler as Ron suggested, let it dry again, and then run it through a drum sander. If the washboarding is too deep and sanding leaves it too thin, you could laminate something to it, or just use a new top.
Ron and Robbie, Thanks for the input. It still doesn't seem logical to me that it would have been the harder and denser dark growth rings that would be depressed. I gave up and sanded everything flat, and am starting over with the shellac wash coat, the colored varnish (I made a mistake, above, they were from International Violin) and then 3 more coats of clear IV oil varnish.