You're right, soda ash is cosmetic only so no worries but I understand your frustration. I used to get it on EVERY batch no matter what I tried. It can be due to many reasons: humidity, using high water, certain FO's/EO's, etc. Alcohol spritz never worked for me. Covering with plastic didn't work for me. The only thing that's made a significant difference is reducing my lye water content. My standard is now 1.8:1 water/lye ratio and the ash is MOSTLY gone. Not always though. I seem to get more ash with floral FO's. I've found if I can't avoid it, it's easier to wash it off after I unmold and before I cut.