I just did one in my sonic cleaner for an old 8hp Briggs. Cleaned inside up real nice on first cycle thru, (mine will only do 30 min at a time) 1st carb that I had to dump the water and simple green mix 3 times before I got this carb clean on the outside too.
it had what looked like a mix of old varnish and sawdust caked on outside that I had to scrub with an old toothbrush between cycles thru the sonic cleaner. usually I get a few carbs thru 1 batch of water + simple green....
I have had carbs with the coating as described by the OP before, I think it is the dirt and **** cleaned off of the carb, that is redeposited on the carb. Change fluid and run it thru 1 more time, you should be good.
I have a 3 liter sonic cleaner, an XMAS present from my son a couple of years ago.... only problem I am having? It isn't big enough for a car carb. and though I have not redone a car carb in a few years, now I find myself with 3 to do.... 2, single barrels off of Slant 6s, and a Carter TQ. the 1 bbls I might be able to do in my cleaner one piece at a time (main body, throttle body and airhorn) but I definitely won't even fit one chunk of that 4 bbl in at any one time.
I used to use the carb dip that eats the skin from my hands back when I used to do more car carbs. anymore, most carbs I do are either Kohler, Briggs and Stratton or Stihl. and they fit in my cleaner just fine.