I use Loctite Marine epoxy with some black colorant mixed in to make it gray.
The set time for that is quite a bit longer so it gives you lots of work time. I brush it on with a paintbrush. I suspect it is goes on thinner and smoother than JB weld but I haven't compared directly.
On the undercut, I will usually start with a Dremel but I then move to sandpaper wrapped around the thick Sharpie. Do like 200 grit I'm incremental to 1000 or so depending on what type of polymer you are working with.