Ok, this has got me stumped
I took a couple of short strokes with my new SVST final cut base bevel guide and noticed the file was contacting the plastic base so I stopped before I did any more damage. Here's what it looked like. You can see the dull area on the top where the file was rubbing the plastic.
So I started to think....what could cause that? Well, it could be that the base isn't flat, so the guide bar resting on the ski base is deeper into the ski than it should be. So I checked again with my true bar in that spot (pic below). Although there is some slight concavity in the center, the parts of the base near the edges (where the guide runs) seem flat enough. So that's not it.
(Note that on the left of the pic below, near the edge, you can see a tiny bit of light creeping through. This is where the file took off some of the base after two light passes with almost no pressure! What a sensitive test!)
Ok, so it's not the base flatness that's at fault. What else could it be. Well, maybe the edge already has a greater bevel than the 1 degree my guide is set for? That might cause it. Although I already checked as best I could with a machinist's bevel protractor that this ski appears to have a 1 degree base bevel, perhaps I goofed, so I checked again with the true bar.
I laid the true bar on the edge so that no light got through between it and the edge, and then measured the gap between the true bar and the base 60 mm away. The gap was right on 1mm, so the base angle is arctan (1/60) = 0.95 degrees, or 1 degree close enough.
So the existing base bevel isn't the problem.
The only other thing I can think of is that the file isn't flat, or the plastic extends up a bit above the inside edge of the metal base edge ....the opposite of being "railed", not sure if there is a word is for that....derailed? . But I couldn't see any evidence of that just by eyeballing it.
Anyway, I stopped messing with the ski before I did any more damage. I'm planning on getting this pair stoneground at the end of the season, so hopefully all will be well after that?