Leon is right. There are several types of MDF available from your
local supplier, the most common being the faced variety which gives it
a more polished and harder appearance on the front and back. This is
from the way the cheap stuff is manufactured.
By the time you are cutting just below the factory surface of your
piece, you will be cutting into a substance that will be very powdery
and will not clear your machine as a cutting or chip. The powdered
material and its embedded resins will eat your blades in no time.
Worse, you would probably hate the junk left behind after planing.
(DAMHIKT). MDF is cheap, go buy the 1/2".