MDF works just fine if you finish it. I used spray lacquer on mine. I drew
a vacuum last Saturday and the piece is still locked down as tight as can
be. Based on my experience, you should incorporate some type of release
valve. I will probably re-design mine so that the vacuum that holds the mdf
to the bench and the vvacuum that holds the workpiece are two separate
lines.


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Don't use MDF - it's much too leaky. Instead make your block of
UHMWPE (ultra high molecular weight polyethylene). It's available
from most woodworking ssources. It's not cheap but it /works/.