Refinish over shellac

[rocket]

Just checking!!! I've got a instrument finished in dewaxed shellac, i loaned it to a dude who i'm pretty sure has left it in a car on a sunny day or in front of a window maybe, anyways it's stuffed the finish, got impressions of the case lining all over it. I intend to rub the finish back but not to bare timber, just enough to give me a good surface again and apply a nitro lacquer. Is that a course of action.
Grateful for informed opinions.
Cheers ,,
Rod.

[Kim]

That should be fine but just be aware that it may not be your mate who caused the issue. The stuff inside most guitar cases is very fine synthetic fluff..if there is 'any' thinners left in the finish of a guitar, including metho, that continues to gas off in direct contact with that stuff, it 'will' break down and leave an imprint. Depending on the finish and the volatility of the thinners, and metho is low so it takes longer to leave, it can be 'months' before it is safe to leave a guitar sitting in a case even inside a house, especially in warmer weather.

Cheers

Kim

[charangohabsburg]

I intend to rub the finish back but not to bare timber, just enough to give me a good surface again and apply a nitro lacquer. Is that a course of action.

Rub back (with alcohol) yes, but not sand back (I think it would be unavoidable to expose bare wood when sanding), or you may get a blotchy top just like someone shows off over there at the OLF. So, if sanding it should be to bare wood.

[simso]

Is the shellac clear or tinted.

You can add another coat and rub it on, it should fix it up okay unless its a tinted shellac mix.

To remove I would use alcohol as well.

You can spray a lacquer over the shellac if it is dewaxed, if its not you can do it as well but you need some silicon addiitves to combat the fish eyes. "trickier" always test spray in an inconspicuos spot

[auscab]

I intend to rub the finish back but not to bare timber, just enough to give me a good surface again and apply a nitro lacquer. Is that a course of action.
Grateful for informed opinions.
Cheers ,,
Rod.

If you want to cut back a french polish job I wouldn't just use alcohol , it does work ,but it also can turn it to sticky jam that is hard to control , alcohol maybe if you want to take it back to bare wood.
you need to cut back with a worn out 400 grit paper and a soloution of linseed oil and mineral turps 20 / 80 ,wipe off , see if you have gone deep enough, if not do a bit more ,when its back flat ,buff it dry, then I would be giving it a thin re polish with shellac .
Another way is, oil 20% turps 70% metho 10% , put it in a bottle and shake. pour a bit on , cut back with a worn out 400 grit , try rubbing with the grain , then buff dry. I use this one for removing later colour jobs from the top of a FP job. It leaves the finish pretty good, not perfect though , you still may have to re polish it a bit. shake the bottle before every aplication.
French polish is great for being able to repair ,if you have the patience .
Why Nitro over the top ?

[kiwigeo]

I've taken a few french polished tops back to bare wood on occasions. Weapons of choice were sandpaper and a cabinet scraper. Using meths is a p in the a and as Rob mentions you often end up with a gooey mess.

[charangohabsburg]

Using meths is a p in the a and as Rob mentions you often end up with a gooey mess.

I've got different experiences. Using alcohol is like reverse french polishing with excess of alcohol. But I imagine that if you flood the wood with alcohol you probably will cause a mess. If you have used another method than french polishing you probably will have a layer of shellack which will be a bit thick to remove with alcohol.

[auscab]

I would agree with you that it works Markus , I just dont think its the best way for Rods problem, I have used it that way on certain jobs , small ones. Usually where I want to half strip back something , put it to one side ,and pick it up later when its dry for inspection, the problem is Metho [alcohol] eats in to the finish quickly and its hard to keep it even ,
With oil and turps its not eating in and you get an even cut back ready straight away for a repolish.
I talk about this from working at this for a living with Antique furniture , and it’s one of the ways the Antique trade repair such damage to French polish jobs. I dont think there is a difference between a French polish job on a table and a guitar.

Rods problem , if he chooses to repolish with shellac is going to be getting a clean finish around the bridge.

I will go to the Anything goes section and post some pics of a nice polish job we have been working on the last few weeks.

[peter.coombe]

Mmm, imprinting is a familiar problem (for me) with shellac finishes and is usually easily fixed. The oil and sandpaper method is good if the damage is severe, but with imprinting you don't need to cut back much at all because the damage is mostly surface damage. Depends on how severe the damage is, and with french polished finishes it can look wrecked, but is easily fixed to like new with a light sand and a few french polishing sessions. Not being able to see the damage makes it difficult, but my experience with imprinting is if you sand it with 1200 wet/dry to remove most of the marks and then french polish with maybe a bit more light sanding in between sessions that will fix it. You can use oil with the 1200 wet/dry to stop it from clogging, or go to 800 to remove more shellac, but if the damage is not so bad oil is not necessary. I use Olive oil. Severe damage is when the shellac cracks and crazes and then needs to be mostly removed. Removing with alcohol is not the best method because you can end up with a gooey mess unless it is just a small piece. I second the oil and sandpaper method for more severe damage.

Shellac finishes are so easily repaired that you will probably spend more time with nitro than if you stay with the shellac. The biggest problem, as Rod said, is to get a clean finish around the bridge.

I had some unwanted practice fixing this exact problem after returning from Port Fairy Folk Festival this year. Two of my mandolins were in black cases in the back of the car and it obviously got too hot. The white Presto case was fine. Why are most instrument cases black, it is the worst possible colour. Anyway, the new owners have no idea the finish was damaged. A light sand, a couple of french polishing sessions and even I can't see the damage.

[auscab]

You, are right there Peter , How severe is the damage ? I am imagining impressions deep enough that need cutting out

Faint marking such as white water marks ,slight differences in reflection ,come out with a dry rubber, charged with mainly metho the only shellac is what is in the rubber from previous polish jobs. maybe with a dab of oil