Too hot (summer) and too thick seems to be the answer.
I removed most of the bubbled shellac by painting on metho, waiting a bit, then wiping off the goop with a rag. A scraper blade can help remove bulk goop on the back (flat), but can "relic" the finish. When the bulk goop was removed, I used a mouse sander with the finest grit (P240) to smooth out any rough patches. Cleaning the sandpaper frequently to remove the shellac dust is a good idea, before it "blobs" and scratches the surface.
I think at that point I painted on some metho to dissolve any remaining bits of goop, but didn't wipe it off; it looked pretty good apart from a couple of dull patches.
My new plan is:
* paint on a single layer of shellac with a foam brush, and let dry.
* paint on some metho to smooth out the shellac (ridge lines, touch up defects etc.)
* sand it back with wet 'n' dry to knock off the ridges (no overall build up - fill the valleys.)
* repeat
* Ideally, cut 'n' polish back with Micromesh pads (1500-12000).
I'm glad that I applied the "white" shellac to the spalted maple front first; any runs of garnet shellac can be carefully sanded off without damaging the veneer. (I may need to apply some more coats to play it safe.)
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