MDF Surfacing

MDF used for vacuum bed:

Is this MDF surfacing a one time process or need to do frequently ? How frequent ? Once a week or once a month or ? Can moisture absorption close the pores in the MDF and make it less porous ?

You generally just have to do it once, primarily to make sure that the bottom of your router is parallel to the top. I've never heard of moisture absorption affecting the efficiency of a vacuum spoilboard but I suppose in areas of extremely high humidity it could be a thing. But I doubt that resurfacing would help much in that case.

If you are holding parts by vacuum through the MDF, you'll need to surface it regularly to keep it smooth, as you'll be cutting into it when you cut your parts, creating grooves that reduce the holding power due to vacuum leakage.
How often depends on how deep you are cutting into the spoilboard, and how much holding power you need. I sometimes surface our spoilboard several times a day. I try to only remove .002-.005" each time.
And no, moisture will not make it less porous.

Thanks for the messages. So from the replies I understand that "moisture does not affect porosity of the MDF" from a vacuum point of view.

So: Looks like We need to do resurfacing of MDF for 2 reasons:
R1) To remove the top and bottom coating layer : to ensure POROSITY of MDF.
R2) To make the spoil board flat w.r.to the CNC axis.

Actually we are using the vacuum bed for foam cutting with a oscillating knife.
We keep a sacrificial open cell foam (styrofoam/EPS) on MDF and then my EPE foam on the EPS.
So we will not be making cuts on the MDF. (unlike a CNC router ).
So my MDF flatness is never affected by the cutting.

Does the MDF still needs periodic resurfacing to maintain POROSITY?
Or : periodic resurfacing is only for maintaining FLATNESS ?

Any thoughts on this ?
Is resurfacing needed to maintain FLATNESS or for POROSITY also ?

Flatness/Thickness, yes.
Porosity, not sure. I wouldn't think so.

MDF surfacing usually only needs to be done once. MDF should offer a smooth and solid surface for a variety of applications once it has been covered and treated correctly with paint or primer. Resurfacing may be necessary depending on usage and moisture exposure. MDF surfaces can be kept in good shape for a long time without needing to be frequently resurfaced if they are properly sealed and maintained.