Since you have the file for the tie strip, why not use it to make a painting mask? I think you can us it to make a negative surface in a new solid, then cut that to expose the tie plates and clips to be painted.
The resin printing isn't quite as precise as say photo etching, so making a mask that fits well enough (without bleed, fits a bunch of flexible strips, etc.) could become a bit problematic. For the amount of strips I anticipate, the pad seems like a 'good enough' approach, at least for now.
What does add steps to the painting process is trying to keep paint out of the areas where the rail will be glued. My solution so far is to stick some .040" square styrene strips into the rail guides before hitting them with the spray cans. A bit crude, but again workable for the amount of track I intend to build, at least in the short term:
Ed