[INM@RLM]

Thank you for that very good spot, Adriano. Both of the following comments are also spot-on to my mind.

That Bf 109 G-2 application deepens the mystery though. We now have two unrelated mentions of what is probably precisely the same protective lacquer (to get very exact, in one place it is described as Isolierlack S 238, in the other it is Isolierlack IS 238), but in neither place is it associated with a specific Fliegerlack-Nummer. For the Fw 200 C the coating is tinted with RLM 65, but on the Bf 109 G-2 it appears to be a clear finish applied to the whole aircraft after both undersurfaces and upper surfaces have already been camouflaged. ("Nach Fertigstellung oben und unten mit Isolierlack IS 238.")

The lacquer recipe is confirmed as belonging to Warnecke & Böhm in the Bf 109 G-2 mention, but Firma Ruth was an acknowledged licensee of Warnecke & Böhm. (Explicitly confirmed in the Fz. Handbuch for the BV 138; Teil 0, Anlage 2 Oberflächenschutzliste, Seite 1.) If we take that section as our guide, Ruth seems to have specialized in production of the smaller-run Wu.B lacquers including those applied to metal surfaces for use in maritime environments.

The same page of the BV 138 Oberflächenschutzliste includes a useful table setting out the earlier company designation for each lacquer used on the 138 with the subsequent Fliegerlack-Nr. Yet there is no mention here of an Isolierlack nor of any Wu.B formulation numbered 238. (Although there is mention of a Wu.B 138 subsequently designated as Fl. 7118.) I've also checked through the 1942 and 1944 editions of Die Flugzeugmaler and found no mentions of an Isolierlack, even in the four-page 1944 reference table of previous designations and subsequent Fliegerlack Nummern.

It will be most interesting to see what this thread turns up, but from the lacquer name it does sound as though we are dealing with a finish conferring specially enhanced sealing properties, hence more expensive and apparently too specialized to have ever been assigned its own Fl.Nr.

As to why this finish might be used on a Fw 200 C but not a BV 138, the later was genuinely an all-metal aircraft.

We are conditioned to think of the Fw 200 as the first recognizably modern four-engine air liner and this is indeed one part of the truth. However, structurally the Fw 200 was the last of an older order of designs with wings outboard of the engines from the main spar to the trailing edge being covered in linen fabric. Thus although it has been claimed outright that the Fw 200 was "das erste viermotorige Langstreckenflugzeug der Welt in Ganzmetallbauweise", this is not technically accurate, and FoWu themselves at the time classed the design as of mixed construction (Gemischtbauweise).
Hence a possible justification for the use of Isolierlack S 238 on the undersides of the Fw 200 C was to seal all undersurfaces, most particularly the very large fabric-covered areas of the outer wings, to ensure that despite wet runways and low-flying over the sea in all Biscay weathers, the fabric-covered portions of the undersurfaces, most exposed to water spray of all sorts, would not become water-logged and begin to rot.