Used a lot of Hartco 425. Make sure you get the versions with paper liner if you want to plotter cut it. The plastic liner allows it to stretch and warp, in other words it doesn't seem to cut well on a plotter. If you're sandblasting sign foam get the highest tack and blow off the dust first for best adhesion. even then i use a stapler and staple the small pieces on. If the mask is narrower than the staple you can clean off the area where the staple when beyond the mask easily after it's blasted.
We never took the time to prime it first because we never had time or available space to wait for it to fully cure and then re-prime the freshly blasted areas.
On really large signs we would lay the mask down and use spray adhesive to secure a pen plot and then hand cut the stencil. At times this is easier than trying to line up 2 separate masks. for smaller detailed areas on signs like this, we would plotter cut certain components and lay them in later. The trees were plotter cut and the rest was typically hand cut depending on the overall size.
We also used to weed the graphic on the sign if i remember correctly, and we never cut the signs to shape until after they were blasted. That way the corners don't get dinged up at the blaster or during transportation.