MIMF

Hi folks,

I am finishing a steel string acoustic guitar with IRW b&s. I've made a shellac pore filling on, giving a very flat surface. I've spay 2 coats of stewmac's water based varnish as usual but it gives me a kind of symetrical white streaks on the back.

any comments are welcome.

Cédric Verglas

Did you use shellac and pumice as the pore fill? If so, it may be that the pumice wasn't thoroughly wetted giving you the white spots where there is dry pumice in the larger pores. The pumice fill step requires that you use only alcohol on the pad. That will dissolve enough of the previous shellac coat to stick everything together and will allow the pumice to be saturated so the white spots don't occur later when the pumice dries.

Sometimes, EIR has mineral streaks in it, but they aren't usually symmetrical and the shellac usually bleeds over enough rosewood coloring to make them disappear. If they don't, I use a brown Solvent marker (like a Sharpie) to color them before I spray.

My guess would be that since the streaks are symmetrical about the center line of the book matched pieces, that it's something inherent in the wood itself. Interesting, but I don't know how to fix it.

Waterbased finishes will NOT "wet" the surface; they simply lay on top of it. Could also be that the WB varnish isn't adhering to the shellac.

You need to strip it off; start with a scraper and notice if the varnish peels-off of the white-ish areas more readily than it does from the darker areas. If it does, then you have an adhesion issue. If it seems to have correctly adhered, remove all of the WB stuff, and give everything a good, wet "french polish" session, without oil if possible. If the surface still looks good after a few days, try the WB topcoats again. If the problem returns, then the WB isn't compatible with the shellac.

Yes I did use pulice & shellac as usual, but I just remind I've used a new brand of alchool. I am going to sand this back to bare wood and check for those whitish things.
I just need to understand.

Thanks.

I am back,

After sanding all varnish, I've tried cleaning the back with alchool, then spraid (?) one coat and the same streaks came back.
I've done the same with acetone and then turpentine as cleaner (to try) and the same problem come back once varnished, at the very same spots.
I'll try Mario's french polish solution...

To be clear, the white streaks only appear when you apply the varnish? That is, when you wet with alcohol, acetone, turpentine or shellac they are not visible, nor can you see any difference in the wood, either visually or in absorption, in the places where the white streaks appear?

That's it pete,

I've found a simple solution: I've brushed 2 coat of the same varnish with the back in an horizontal position, let dry over night and it's OK at 99% (the 1% remaining is just because I know where they were )
It doesn't expain me why but now I can go forward...
If you have any thought why, you're welcome.

Mineral deposits in the wood, or something was spilled on the wood and then the two halves were faced together, transferring the "stuff" in a symmetrical pattern. Probably mineral deposit.

Thanks

I was using some scrape from this back for a repare job on another instrument, and I've found small white mineral material in some pore... so my issue was from Mineral deposit......you get true!

Where does it come from? and how to treat this ?

The tree pulls-up minerals as it grows...

Usually, just wipe it with alcohol a few times to smear some of the oils and color into the white pores, then proceed with the pore fill and finish. In bad cases, you need to use a pigmented(colored) filler.

I use an appropriately colored Sharpie.

beware some permanent marker inks can turn different color under finish. I used a brown Sharpie to touch up a small area on a mandolin some years ago, only to see it turn fluorescent green when lacquer was applied. Quite a shock and disappointment at the time. Test on scrap.