Masking lines on rotor blades?

That is so simple I should have thought of that myself!

Thanks, I'll give that a go.

This sounds like a perfect place for a jig.  Trace the outline of the blade tip area on a sheet of white styrene.  Now use a square or protractor to lay out a line perpendicular to one of the sides at the right point.  Extend this line at least a quarter inch- maybe even a half inch- beyond the blade outline.

Now clamp that jig to each blade end and lay a piece of tape across at the line you made.