Look up nitrocellulose (nail polish) for resistance to your particular motor fuels or actually perform a test. In the States they used to use nail polish. I think that it is fairly resistant to regular gasoline but not at all to alcohol. YMMV.
Any time that you rebuild a carb and replace the welch plug you should probably do a leak test by pressurizing the venturi with a home made Stihl adapter as shown in the attachment
Excellent attachments thanks!
No worries with Ethanol fuel here. I don’t like the idea of any sealant pasted all over the plug regardless though.
The best of both worlds - I use nail polish and the brush to spread a very thin layer onto grease proof paper, then take the welch plug (you know how it will semi stick to your finger if you press firmly on the top of it) and dab it right onto that nail polish and then transfer it to its seat in the carb.
It’s so little that it’s not visible by eye, but enough to take up any minor imperfections between the plug and carb contact surface.
I have been thinking of ways to test the validity of the welch plug. I’ll run it first, but any issues I’ll pop out the metering lever and needle assembly, block the H and L drilling’s in the metering chamber floor with blutack and then put it back together. Then I’ll fill it with a few psi of air from the fuel barb and check for leaks. You could spray soapy water through the venturi.