Official Luthiers Forum!

hi,

This top was prepared as follows
sealed with shelack it was almost FP done, then thin coat of PU lacquer over it.
After year and so owner wants to refinish the top...because there was several deep scratch from thumb nail and region on treble side of lower bout was completely rub of lacquer to the bare wood.
well I sand all top to the wood , wipe it again with schelac and spray it and top show that decolorizations between rosete and bridge and some other place which I can’t avoid.
pic show situation after spraying and wet sanding(1000) second time

Are there some tips, tricks and rules for refinishing cedar top to get even color?

I can only guess but I think that when you have sanded you have removed oxidised wood. That will make the Top look very patchy. It's virtually impossible to sand an old(ish) Top and get even colour.
If this is the situation there are only a couple of remedies. Finish it and let time do the rest, it will take years for the colour to become even.
Strip it again and expose the bare wood to UV. In a light cabinet it might take 3 or 4 days. Exposed to strong but indirect sunlight it might take 3 or 4 weeks, probably longer in winter. You really need to get ALL the finish off first though. Once you have the lacquer coats off I would try to remove the Shellac with alcohol, very carefully, watching that any inlays will not bleed into each other or into the Top.
Personally I think the mistake may have been in removing the original finish. The vast majority of times you can address problems by retouch or overcoating. A refinish is an extreme as far as I'm concerned. . . . but you are where you are.

I also would not re-finish to fix a few scratches. I don't think the problem is oxidation related. Wood cannot oxidize under a finish, no access to oxygen. Could be UV related, but Cedar is not known to be highly photoreactive. A Lot of the color shift that happens over time is in the finish (on most conifers anyway). Nitro ambers a good bit with time. Most other finishes do to some degree as well. It looks like the surface was not sanded all the way back to white. Did you check it with a wipe of spirits before sealing? spots should have shown up then. When shellac is used on bare wood it actually penetrates into the wood a little bit and grabs onto the fibers. Locks in so strong it will seal in anything, sap from knots, smoke stains, ink, anything. That's why people like it as a base coat. It can be real difficult to remove. Lacquers and a lot of other finishes on the other hand just lay on the surface, requiring a rougher surface to grab onto. A vinyl sealer instead of shellac may have helped by bridging the bare wood better, but I think it would have still been noticeable.


So what to do now? I would take the wood back entirely to white. Starting with a good chemical stripper. Be careful, or you will wind up re-finishing the whole guitar. Masking tape is no insurance when working with chemical strippers. The tape is on top of the existing finish and the stripper will eat it right out from under the tape. Small pools of stripper and a card scraper works nicely. Maybe some maroon or grey scotchbrite soaked with stripper would help. Make sure to test your stripper to be sure it does not eat scotchbrite before using it, that would cause a disaster. Don't use steel wool. Work small areas at a time, very carefully until the top is done. Sand the top lightly and check with a wipe of alcohol or naphtha. Any spots that show when wet will need more attention. Depending how much French polishing you did and how hard the shellac was packed into the wood, you may not be able to get back to white without getting to aggressive on the soft Cedar. In that case you may need to follow your original process again to eliminate the problem entirely.

Hope this helps.

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Michael and Brian thanks...
only what I know is that if somebody come to my door with cedar top guitar for refinishing will be shutted

Brian I'll follow you recommendations so will see...