Splitboard inner sidewalls

[antoxasam]

Hi guys. I have a question about splitboard: how do I sand the inside sidewalls of a splitboard so they are perpendicular to the base? I use a belt sander to sand the sides of a snowboard. I just grind the sidewall until the edge is clear, and then I go through the router at an angle of 15 degrees, removing any imperfections:). But if I sand the inner sidewalls down to the edge in this way, I won't be able to align the sidewall at 90 degrees to the base. Share your experience, please. Thank you in advance. It is my belt sander.