Shellac bald spot / dry spot / dull spot

[Steve1]

I have been putting super blond shellac, 1.5 lb cut on sapele recently.
Finished quite a bit of sapale actually, just fine.
But when doing the doors recently, the shellac developed a "dry spot".
I keep putting more coats on the dry spot, but it looks as if I put nothing.

I searched it on the internet, and did not find much.  A couple of suggestions that the shellac may have been overworked.  That is certainly possible -- first coat 2.0 lb cut, went on too heavy (but no bald spots) and I rubbed out a few heavy spots.

Last night I cleaned the spot with alcohol, then this morning sanded it and put 2 coats of shellac on the spot.  See photo below -- looks like I put nothing.

What happened here, and what is proper remedy ?

[attachimg=1]

 

[Packard]

My initial thought was perhaps some wax or silicone lubricant contaminated that spot. 

Since luthiers still use a lot of shellac (and lacquer) I searched there and sure enough this apparently identical issue was addressed at one site.  I did not read all the posts, but perhaps you can find out more there.
http://www.mimf.com/phpbb/viewtopic.php?t=3564

This is the problem they were discussing:

 

[Crazyraceguy]

Also, possibly wood glue? If someone got a little wild on that joint right next to it, then smeared it around, maybe with the usual "damp rag"?
A little sanding or card scraping and try again? see what happens

 

[Steve1]

Suppose I could have got some glue near there when cleaning up the joint, but that area was fully accessible in my glue-up fixture, so most likely got cleaned up well.

I kept applying shellac.  (Hoping sooner or later it would stop magically disappearing)  Mixed some more flakes into my shellac to make it a little heavier, and eventually a full film covered the bald spot, and I put a few more layers on to give it some thickness.

But I sure would like to understand it better.

 

[waho6o9]

That's why I apply a shellac wash coat before glue up. I think it's glue as well.
View attachment 1

 

[woodferret]

I'm actually thinking localized blushing.  Probably some water spots getting pulled into the alcohol and getting trapped.  Next time, just daub it with extra alcohol, don't wipe and let it sit to evaporate through.

 

[Steve1]

I'm actually thinking localized blushing. 

That's a plausible hypothesis.
I wouldn't be surprised if the evaporating alcohol could condense water from the air.

I tried replicating that on a couple of scraps.  On one, I wet the wood in a spot before applying shellac.  On the other, I dabbed a bit of water between coats.
The first one with the wet wood, did show a somewhat similar spot, but went away after a few more coats.  The second piece, with a bit of water between coats did not show a spot at all.
But perhaps I didn't have the conditions just right.

 

[Steve1]

I thought I had it.

Sanded shellac a bit and put a coat of AquaCoat on it.
Then I saw this.
At least it's getting smaller.  [smile]

Edit:  I think I got it fixed (again), but darn, I have a camera microscope, but didn't think to look at it under the microscope.  I look at it now under the scope, and it all looks uniform.

[attachimg=1]