I'm doing my own stairs remod - long story short, I'm regretting it. It looked deceptively easy, but I've regretted it and I have a bunch of little errors that are adding up.
One such mistake is that the skirt board is in, can't be removed and it protrudes from the wall by a little less than 1/4 of an inch.
How would someone who knows what they're doing try to get rid of this protrusion so it's flush with the wall it's against? I'm afraid sawing will be sloppy (especially if I do it). Would a belt sander work?
This isn't too tough. Make your life easier by finding a plinth block that coordinates with your rail system or other nearby trim components:
Use whatever tools you have on hand to trim the skirt back about 3/4" from the wall corner. Fit the plinth block over the end of the skirt to act as a cap. This way you don't need a perfect cut on the skirt, and you don't need a miter return into the wall.
A technique I've used for this is to rabbet the back corner of the plinth block using a table saw. Run from the bottom upward, and stop short of the top of the block. This leaves a curved notch that can fit the top profile of the skirt rather nicely.
For fine control of cutting away a surface as described, you'll find that an oscillating cutting/sanding tool will give you the best results. There are far too many brands to select a single one here, but a search will give you plenty from which to choose.
The cutting edge of such a tool can be controlled well enough to remove a sixteenth of an inch at a time, or use a coarser tooth blade to remove more material in a shorter time. Accordingly, you can follow it up with the sanding tool to prepare the surface for finishing.