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I have a witness line in my French polish repair.
I sanded level a one year old sprayed shellac finish on the lower bout where arm contact had caused some crazing. Then I built back up the finish in that area French polishing with Walnut oil. After two (?) weeks I sanded level up to 2500 w/ naphtha as a lube and have a witness line that nothing seems to remove. The line looks like blush but I have introduced no moisture. Could this be from trapped oil? Insufficient dry time?
I was under the impression that a witness line was impossible with shellac on shellac. But there it is.
Is it necessary to soften the finish some with alcohol before applying new layers like lacquer?
Unfortunately I have a dial up connection so am unable to post photos. Any ideas?

that's a bummer

Did you sand down to wood? If so, you probably introduced some oil to the wood that wasn't present in the sprayed finish that you did. That could be causing your troubles.

I'd suggest a solution, but I don't have any that you want to hear about.

Dave

It’s o.k. Dave , I can take it. What’s your thought for a solution?
I definitely did not get to bare wood. I am familiar with the look of oil bleeding under the finish. That is why I used naphtha instead of soapy water. I was afraid moisture might make it down through the crazing cracks to the wood. I have seen oil make it through cracks in the finish but this is a dark redwood top so I thought I wood risk it. If oil in the cracks was the issue, I would expect it to see the problem over the whole repaired area not just at the intersection. It really does look like a classic witness line. This is a texture on the surface at the junction between the repair build up and the old surface. Further sanding has no effect. I think all I can try is working the junction more with almost straight alcohol. It’s weird.

Strange problem. I can only think that the new shellac is not dissolving the old or that the Walnut oil (being a drying oil) has formed a layer of it's own. Just a wild guess though.

Shellac has some strange properties, and one is that it tends to darken the area around a repair. To get a repair to not show, you have to work a much larger area, and pretend the area that is damaged isn't there. When I have re-worked areas in the lower bout where the arm has caused a break down in the finish, I have just re-FP'd without much other preparation, and it seems to work pretty well. If you sanded that area, then it is hard to get it to build back evenly with the other surfaces. I don't know where Michael Dale Payne is when you need him!

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I had a sinking thought, I may have done this repair while I was struggling with the ,”dirty wool roving” issue. I tried washing my wool out with dish soap thoroughly but still had no luck so I switched back to gauze padding . I may have added good layers on top that now I have sanded through.
Their just is not an emoticon for exasperation big enough! I am ready to start reaping the benefits of shellac’s “ease of reparability” soon.
Much re-melting with straight alcahol has done little. I think I will try and go down as far as I need to without getting to bare wood. At least it’s Redwood and super blond color, I’m not having to work at color evenness on top of it all. Sorry I'm dragging you guys through this with me. catharsis?

I've seen witness lines in FP when I've used walnut oil as well. I have to wonder if the drying oil may be reacting with the shellac and forming a less soluble product, like an oil varnish.

I no longer use oil in FP work, it does take some practice to work large areas without it but for a small spot repair it sounds as if one may be much better off avoiding it.
The coated Gold paper (LMI) is quite effective for level sanding dry. I think the use of lubricant prevents me from seeing what is happening to the surface and I don't know whether there might be some contamination from this especially if one goes through to bare wood.
I've noticed too that old FP is no longer really clear so that when you do a spot repair on newly added wood the repair can often take on a brighter or "darker" appearance.
Spot repair of tops is tricky no matter what you use. If the FP is only a year old I'd take it off with alcohol and redo.

I took off the repaired French polish. The witness line shrank to nothing like a topo line, indicating the entire repair build up was funky. The shellac was the same as the original sprayed stuff so the funk was with the Walnut oil as Mr. Caruth speculated, or the still dirty wool roving. I will now try building back with olive oil and a gauze pad. The only problem is I switched to roving and walnut oil in anticipation of Michael Payne’s class in May. How will I show my face if this works?

Thanks’ for your input David Laplante, I missed your post before mine.
I too sand dry as much as possible for visibility’s sake switching to wet sanding only for the grits that put the shine on. P1000 sterate coated paper is the finest that seems effective dry. I might have suspected the sterate could be involved in my problem, but I used the 3m gold papers leveling the original sprayed coats with no issues.
I embarked on this repair partly because the dark Redwood top removed much of the shading difficulties.
I have heard others talk of French polishing without oil. It’s very intriguing but I can only manage multiple wipes with a pause in between that way, not the continuous movement I use with oil. Is that your method?

Yes, straight pulls in an overlapping pattern.
On a back for instance, I would start at the edge of the lower bout moving parallel to the center line across the entire width. The cut of the shellac plus the degree of "charge" to the pad controls the ability to keep it moving nicely.
By the time I've reached the opposite edge, I can go back and repeat. You have to have discipline not to go back to where you just pulled as the pad may "drag".
This technique requires more discipline for spot repairs as you do have to pause for a moment between series of pulls.
I use 320 then 400 during the bodying phase and don't go beyond 600 dry with the Gold paper as at that point I'm glazing (hard totally squeezed out pad, very thin polish and multple passes to build back the shine from the matte sanded surface). From that point I'll rub out with Mequires heavy cut, fine cut and glaze to refine and soften the surface luster.
This is certainly not the traditional technique (though it is quite similar in the final stages) but it produces an extremely thin fnish and a very refined surface .

Thanks so much for your help Mr. Laplante,
that 's the first time I've heard it articulated clearly. I have been able to do the oil less method as you describe. It's an extremely light, sensitive touch to feel the right speed and pressure, and a just slightly wetter pad. I just was under the impression people were able to somehow do this with a tight pattern as w/ oil. The more flashed off, the silkier the glide, I find. So far so good. No witness line this time. I'm inspired! back to the shop for a couple more dry, disciplined, coats. Dry, glide, dry, glide, dry, glide.........................

I’m not sure I can give up my addiction to that supper flat shine, sanding up through the highest grits (Imperial 2500) and buffing. Too many years perfecting Lacquer on steel strings. I know it looks incongruent on a classical. Maybe a twelve step program?