My TS outfeed 'bench' is mdf and I usually re-coat it whenever I have a little bit of finish left over. I use mostly wipe-on finishes and now have layers of sketches, scribbled notes etc. buried within the finish. There's even the old miter slots from switching TS's (I simply filled in the old slots and re-routed new ones to match the new saw).
For long term protection epoxy is probably best but products like West System aren't cheap; poly is probably the best balance between cost/protection. I use whatever's left over because I'm not super concerned with the surface and I like to be able to draw on it.