I use a little brass chisel type tool, bit of brass bar only 0.25 inch wide, filed to a single bevel edge. Harder than copper, not so hard it will scratch even not-quite-hardened-enough steel.
One needs to be careful with Vaseline for masking as it is very easy to inadvertently transfer it to somewhere you want the glue to stick. Chances are that a MS is doing hidden tangs with guards, so epoxy contamination is the problem, but not much risk of grease contamination of the tang. Full tangs pose a bit more problem where the scale meets the ricasso. For those I use brown packing tape, cut with a scalpel to trace the leading edge of the scale. I also tape the end of the scale, so I can wipe epoxy and not worry about it changing the finish on that pre-finished bit of handle.