I know that is required by UL when reducing washers are incorporated:
The following information comes from UL 514B;
8.34 Reducing washers resistance test
8.34.1 ? reducing washers shall be assembled to threaded conduit and unpainted steel plates as illustrated in Figure 16, and then subjected to the resistance test described in Clause 8.9. As a result of the test, the voltage drop shall not be greater than 10 millivolts. The samples shall then comply with the current test described in Clause 8.10.
I was not aware it was required on standard locknuts.
To take it one step further, cat. code QCRV states that reducing washers are listed for grounding when installed per NEC. It is not mentioned, but (assumed?) the installer will remove paint.
Maybe it's in the installation instructions, but who publishes those for reducing washers? (sarcasm)
Is it assumed the user will remove paint for locknuts also?
If you compare statements in DWTT and QCRV under grounding neither tell you to remove paint. The standards tell the manufacturer how to pass the test for reducing washers, but are generally unavailable to the public, like the guide info is.
Even cat. code KDER has no help. It talks of grounding locknuts, but those are ones with a set screw. You EMT connector has probably not been investigated with that particular locknut.
Cat. code DWTT does have a blanket statement that: All male threaded fittings and nipples have only been investigated for use with locknuts.
Maybe that means any locknut
a search on the entire UL website yields no hits for "remove paint" that pertains to locknuts or conduit fittings.
Only one product guide info that I could find has any mention about paint and that is Victaulic couplings. "The pipe is to be grooved or rolled in accordance with the manufacturer's installation instructions. Couplings are protected by galvanized coating or may be painted." File: KDER.EX1820
So looks to be up to local AHJ unless more info is forthcoming from UL
To access what I referenced use this link:
http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.html