So.. the 3/32" thick block did look better once I got a larger patch of it done but still looked too clean or straight or something or maybe just too much like little pieces of wood glued to plywood.
So... goop to the rescue. Slather it on thick & wedge it down into all the cracks and crevices.
Then, I used a wet towel to wipe off the inxs. Use a scale concrete mortar smoothing device, like a wet finger, to smooth out the grooves.
Let it dry & trim out the windows & any offending basswood block.
I then painted my "goop mortar" with a light gray.
Once that dries, or if your impatient like me, before it dries, hit the top with off white using a little trim paint roller.
As with anything repetitively tedious, you refine your technique so it makes sense to start in an area in the back that you won’t see so that by the time you get to the front, you’ve learned from your mistakes. I didn’t do that. But… In my own defense, goop is spackle & I’m using latex interior paint so it’s pretty reversible.
I need to go back and put a second layer of spackle on and work on getting smoother joints. Also, I found that painting the entire thing gray & then just a very light rolling of white is key. It doesn’t take much extra paint on the roller to blob down in the mortar.