How large is the pocket?
What’s the purpose and why do you need it to be hard & smooth?
I’d agree with Will, the downcut won’t be helping much as pushing the fluff / chips down into the MDF will make more fluff. Also, you need sharp bits in MDF to get clean edges.
I use one of these for surfacing MDF, it is a ‘bottom cleaning’ bit with faces on the bottom to cleanly surface the bottom of the cut
I run it at 20kRPM, 2,500mm/min feed and 0.25-1mm depth of cut and get a clean finish. There are other more specialist cutters which you can also use.
To get really clean crispy surfaces and ‘hard points’ for probing zero, bolt counterbores or inserting dowel pins for location I do the first roughing passes and then soak low viscosity CA glue or Tite Bond into the MDF to make a composite before finish passes, this gives reasonably predictable and stable hard points to work from.
