[ger21]

Depends on the pump(s) you are using.
We use regular old MDF, with our two 10HP Becker pumps.
If you are using a vacuum cleaner motor, you might want lightweight MDF.

 

[ger21]

Ask for super refined mdf.

Are you trying to pull vacuum through both the spoilboard and a sheet of 1/4" MDF? 1/4" MDF is much more dense than 3/4".