Mamiya-Sekor 150mm f/3.5 for 645

[Neil Grant]

so, in an idle moment I decide to open up one of these for cleaning. I notice that the edge of the front element, which is not part of a sub-assembly, is completely un-painted. Is that normal? I thought the edges of all camera lenses would be treated with a lick of black paint. I'm tempted to add some paint - what should I use??