Occasionally at our shop we have a need to build a project with dissimilar adjacent finishes. It may be only a different stain colour but at other times it’s a radical shift in the look (eg. paint vs clear-coat).
This has always been a problem in the past. It required ultra careful masking by our finisher and yet it still failed in small ways. The straight lines were not always perfect and the colours would often bleed under the edge by capillary action.
The only people that I have seen do this taping flawlessly on a routine basis are painters doing daily auto body work. Their stuff can be superb, though to be fair to ourselves, they are only dealing with airborne materials.
My technique may have been discovered by others but I’ll throw it out anyway. It’s a good trick.
To make this work you must have in your kit a wheel type marking gauge, such as the one sold by Veritas.
Mask off the break line with a good thick sticky tape (we like 3M’s 203) and roll it down. Set your sharp wheel gauge to the require depth and cut the line with a slight rolling motion, end to end. Remove the tape on the side to be finished first and do your worst.
What you have done in this process is created a very thin V groove at the break point and in addition, you have forced the adhesive tape down into that groove, providing a seal.
After the first finish side is complete, strip off and re-tape the joint, and with your preserved wheel setting, cut again, then finish the other side and remove the tape. You will be left with a microscopic, slightly tactile line of top-coat material that can be cut away cleanly with a dead sharp chisel or razor blade.
I have done this on curved work as well though it requires a small curved fence block double sided taped to the gauge and a rather steady hand.
-- firstname.lastname@example.org : dovetail free since '53, critiques always welcome.