plastering over wood board

In theory, provided you apply a primer/sealer like PVA or BlueGrit to that surface, and also provided there aren't any joints in it that can move, then you probably will 'get away' with skimming it.

But to be safe the simplest solution is to over-board it with 9.5mm plasterboard first - no need to remove the MDF.

 

When you say a section is MDF, is there a finished wall surface either side ?

Just wondering if MDF has been recessed say 3/4 mm so plaster will finish level with surrounding wall

If you overboard with PB, then skim, what's the finished level like in relation to either side

A photo may well help here

 

Expanded stainless rib lath nailed over the boarding and then plaster is another option

 

I had loose plaster where a 6 inch wooden joist supported the upper wall going up the stairs.
Covered it with chicken wire fastened with wire staples and re-plastered and no problems since.

 

Is the adjoining piece mdf? Anyway, if you want to plaster, overboard with PB and fix with 25mm screws or grip fill, put an angle bead on the corner and re-plaster.

 

Is the MDF section just that small piece in the picture, looks like a tile, maybe 250 X 150 mm

If so, what's the reason it's been placed there - is there a hole in the wall

If yes and yes to above, it's gonna be easier to remove and patch in flush with a piece of plaster board, neaten up edges with filler then you can decorate

 

If the rest of the wall is in an OK condition, then your making a lot of extra work by wanting to plaster the whole wall, including working around the window reveal

If it's just that damaged section, why not just repair that. Also if your planning on leaving the piece of MDF on and plastering over complete wall, how thick as the MDF - 9/12 mm

Plaster is usually applied at around 3 mm depth, unless you bond out the wall first

Just seems like a lot of work when essentially you have a hole in the wall

 

I was referring to the bit to the left...presume that's the soil pipe boxed in. I would remove that bit of mdf and overboard with PB. If the other bit of boxing in is PB, then scratch up and pva. If mdf then overboard. Either way, put on an angle bead and plaster both surfaces...job done.