Soft finish marking

[Dave Greorgeson]

:no:I've just finished restoring a mandolin and have put about 15 coats (rubbed) of tinted shellac and the same in super blond shellac, followed by a light polishing with a rubbing compond and auto polish, but even after 4 weeks the finish is still marking when it is left resting on anything... even the vevet in the case is marking it.. Anyone out there got any ideas as to why this is happening, would a hard finish shellac be the cure?

[KJ401]

Most likely causes would be either your shellac is old or the metho used had a high water content. Once shellac is dissolved, it has a short life, particularly in humid climes. Shellac flake will also go off, especially if exposed to light (which is why buying shellac in clear plastic bags from hardware stores is not a good idea). Always use denatured alcohol rather than metho and keep decanting it into smaller containers as you use it to minimise air contact.

The other possibility is that your shellac wasn't the right cut - it was too thick. If the shellac is too thick, and the weather hot & really humid, you can get problems with solvent trap, which can cause softness and later crazing.

I'm really sorry to have to say this, but if it's still that soft, it will have to come off and be done again. You could try padding it with a rubber just damp with some denatured alcohol - just damp. Then leave it for a couple of days out of the case ( it could be sweating in the case too). You don't need to go to a hard shellac finish - just make sure you use fresh shellac and denatured alcohol. Hard shellac finishes will cause problems for the next person to restore your mandolin in 50 years time...

[Dave Greorgeson]

Many thanks KJ401. Thats disheartening to hear, however I had expected as much, since the final coats were done in high humidity, and yes, I think maybe the mix was a little strong. I didn't use de-natured but used 95% metho. Because I haven't had this result before, I suspect it to be mainly humidity...I live in Far North Qld.
Were you suggesting a weak mix with de-natured or just de-natured on it's own?
Dave.

[Dave Greorgeson]

Also I had purchased the blond shellac on line, in granular form, it disolved well in a short period to a clear mix. as you state ..It was in a clear plastic bag. however I decanted it to an airtight container and keep it in a cupboard, the same for mixes.

[KJ401]

As your shellac dissolved well, then it probably was the combination of high humidity and your shellac being too heavy a cut. Just try padding with straight denatured - no shellac, though I would use the rubber I had during the shellac coats - rinse it thoroughly with denatured, squeeze and squeeze again until it is "just damp". Touch it to your cheek and if it feels like a light peck on the cheek, then that's "just damp". Work the finish with quite a firm stroke - you're aiming to very slightly activate the finish and pull it, hoping to give the trapped solvent a chance to escape. A hot low humidity day would help.

[Dave Greorgeson]

Thanks for all the sound advice KJ401. It's so humid here just now, I guess I will have to leave things as is until the drier months, at which time I will put your advice into practice and hope for a cure without more drastic action. Cheers, Dave.

[haugh richard]

As your shellac dissolved well, then it probably was the combination of high humidity and your shellac being too heavy a cut. Just try padding with straight denatured - no shellac, though I would use the rubber I had during the shellac coats - rinse it thoroughly with denatured, squeeze and squeeze again until it is "just damp". Touch it to your cheek and if it feels like a light peck on the cheek, then that's "just damp". Work the finish with quite a firm stroke - you're aiming to very slightly activate the finish and pull it, hoping to give the trapped solvent a chance to escape. A hot low humidity day would help.

.........