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I'm restoring a multimeter from the 1950s and I was wondering what was used as paint-on insulation/conformal coating. It's the yellow/brownish stuff smeared on the screws and leads in the photo. I suspect it is shellac, was this commonly used in the 1950s in electronics?

It is not greasy and quite hard. It will melt a bit with a soldering iron.

example of brown/yellow paint-on stuff

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  • \$\begingroup\$ does it have a greasy consistency? If so, it's contact grease. \$\endgroup\$ – Marcus Müller Aug 8 '20 at 10:35
  • \$\begingroup\$ It isn't really insulation. Think of it as a kind of conformal coating, intended to keep moisture and other surface contamination away from the circuit connections. \$\endgroup\$ – Dave Tweed Aug 8 '20 at 10:44
  • \$\begingroup\$ @MarcusMüller no, it's not greasy \$\endgroup\$ – rve Aug 8 '20 at 10:49
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    \$\begingroup\$ Shellac I believe. \$\endgroup\$ – Andy aka Aug 8 '20 at 11:16
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Photographs are not reliable color indicators. And there are greases near that color.

But that really looks like the signature color of GE’s Glyptal, which is an insulating paint specifically made for electrical equipment. Glyptal comes in very basic colors like black and white, but that particular burgundy is their signature.

Given the 1950s era, Glyptal makes a world of sense, since the product was in its heyday then.

I’ve never had occasion to stick a soldering iron on Glyptal, so I don’t know what it does. But it does not like to burn, unlike normal paint.

Glyptal is readily available today, and is used for such purposes. Also, auto racers paint the inside of engine blocks to capture any remaining casting sand, so it doesn’t wind up in the lubricating oil. I suspect they are after the “non-flammable” characteristic, not its dielectric strength.

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shellac (or some other type of varnish) for a hard heat resistant coating and paraffin wax for a softer meltable coating.

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For many applications, a fast-drying varnish or enamel was used. The base was whatever volatile organic solvent was available at the time. The material left behind when the solvent dried was whatever plastic material was available. Nitrocellulose was developed in the mid 1800's and used as a plastic or plastic coating for many purposes.

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  • \$\begingroup\$ By the 1950s, nitrocellulose had lost most of its applications to, err, more stable compounds. It may still have been used where explosives were required, but not as concoat. \$\endgroup\$ – Brian Drummond Aug 8 '20 at 22:20

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