| Agreed. MDF is a bit better, dimensionally stable but will succumb to
| water after a while. It swells and not necessarily uniformly.
| I have a few pieces that were on the floor under my table saw and got
| wet from rain under the door. 3/4" is now closer to 7/8 and wavy on
| If you can keep it dry, it is OK.
| The gold standard would be the pure plastic "Starboard"/"Azek" type
| Trex is somewhere in between. None of them have the rigidity of a good
I'm assuming he's not talking about outdoor
use, so I wonder why Trex or PVC or vinyl would
be comparable. I also wonder what aspects you
value to call plastic "the gold standard". It's
ugly outside... It's even more ugly inside... It's
relatively expensive... Did I mention it's ugly?
OK for something like fascia and soffets, if your
house is already ugly, but mostly it only seems
appropriate for utility usage where water is a
problem, like a ground-contact trim board.
I use MDF for laminate substrate, on top of fir
plywood. It has little shear strength, absorbs
water, and chips easily, as it's really a composite
of paper sheets. But it's very dimensionally stable
with good compressive strength -- just the thing
for laminate countertops. I can't think of much
else that it's good for. I xometimes buy cabinet doors
from a shop that likes to use MDF panels. They'll
use birch ply if I fight with them, but they don't like
to. To my mind 1/4" MDF is not a good choice
for that. If it gets wet or gets hit the panel will
have to be replaced.
I once built bookshelves for an architect's office
of MDF. They wanted an "honest" material, with
no seams and no edge banding. :) That worked
OK. The shelves were all dadoed and the whole
was painted with oil-base trim paint. The shelves
didn't have long spans, so the MDF was strong
enough for the use. But I chipped one corner of
the bottom delivering them. It hit the sidewalk and
knocked a chunk off. Very delicate in that respect.