There is another entry for this product (Isolierlack S 238) here:
http://www.wwiiaircraftperformance.o...42-german.html
Translation:
After completion, top bottom with insulating varnish S 238 Farbennersteller (Warneke and Böhm)
I contacted them (
https://www.wb-coatings.de/kontakt-en-gb/) can give us a glimpse of what it was (seems to be some kind of insulating varnish to preserve corrosion by sea water or intemperies?)
Am not a painting specialist but would like to know how it worked and how it was applied over the airplanes.
NICE THREAD by the WAY! Congratulations
A.