Was making up a jig yesterday and found after the glue had dried that my miter slider was a little oversized. With the alternatives being to pry it apart, which never goes well with MDF, or start over from scratch, I went after the slider with a shoulder plane, expecting dismal results. To my surprise the stuff planes beautifully, with the only downside being really continuous shavings long enough to form a solid mass in the throat of the plane. I'm sure it didn't do the edge any good, but that can be resharpened.
I'd never tried taking a hand tool to MDF before--now I'm curious.
I did learn not to secure MDF miter sliders with flat head screws--they'll split.