As I've mentioned a few times in other threads I use diamond pastes on iron and/or mild steel plates for some lapping jobs, particularly flattening chisel backs.
After seeing all of the posts from people suggesting MDF as a substrate for diamond paste I decided to give it a go. I tried every paste I have on hand: Norton water-based paste (45, 15, and 0.25 um), DMT oil-based paste (6 and 1 um) and PSI oil-based paste (45, 15, 6, and 1 um).
I was initially impressed with diamond on MDF, but noticed that the cutting started to slow down markedly within the first minute. The resulting surface finish was also finer than I would expect from the same size diamond particles on iron. I suspect that the diamond particles are receding into the relatively soft MDF such that their cut depth decreases - this is a known issue with basically all abrasives on compressible media (paper, cloth, wood, MDF, etc). This effect was present for all of the pastes I tested, but maybe a little worse for the Norton aqueous ones. This may be related to why people get such great results with the ubiquitous green lapping compound on strops even though it contains much larger calcinated alumina grains along with the 0.5 micron ZrO2. If the alumina particles initially cut fast but then recede, then you'd get nice speed at the start and a smooth finish at the end. I'm not a fan of that behavior with something as costly as diamond paste though.
Once the cutting slowed down I checked flatness with a straightedge, and found that I could see light where I'd been flattening my chisel. It wasn't severe (maybe 1/2 mil at the point when I stopped) but it was a lot more degradation than I'm used to seeing with grey iron or mild steel. Again the aqueous pastes seemed a little worse, perhaps reflecting MDF's hygroscopic nature. This isn't an automatic showstopper because MDF is cheap and plentiful, though as with the deceleration concern cost is an issue - In order for diamond paste to be economical I need to be able to use a given charge of paste for a long time.
The MDF does have some compliance and therefore does better when you want to polish a very slightly out-of-flat surface. Steel and iron are totally unforgiving that way - you have to get it dead flat on a coarse paste before you try to polish.
All in all I'll be sticking with iron and mild steel honing plates.