Up until today I was under the impression that MDF is pretty darn flat âŠ.
I was surfacing a chunk of MDF for my vacuum table build, I expected it to be essentially flat but the sheet that was 25â by about 45â required almost a mm of shaving before I deemed it to be flat. Surprised the heck out of me âŠ.
A wise man once said: âWhen you assume you will get the best gift on your list, chances are your Crhistmass is gonna suck.. again..â
Science dictates that when you introduce something hot or cool verses something that is not, the universe will make sure that your hot cool stuff will end up being not.
Entropy dictated your Altmill isnât straight.
Canât deny the science here.
Lol. Gotta remember that one.
Iâve recently noticed that more and more poorer quality MDF products being sold - especially in the big box stores. But, what youâre reporting isnât necessarily caused by inferior product⊠it may also be improper work methods. Let me explainâŠ
1. Non-flat substrate or improper fastening to a flat substrate. Iâve seen this happen quite frequently, where perfectly flat MDF sheetgoods are laid on non-flat surfaces (it may be a frame or a sub-assembly), then firmly screwed down to the irregular surface. Similarly, Iâve seen perfectly flat MDF sheetgoods haphazardly fastened to a flat surface.
2. Moisture causing warpage. This happens when MDF is allowed to absorb moisture (either an overly damp workspace or a sweating pipe above the MDF, dripping water onto it), causing it to swell and the glue in the panel fails, resulting in warped panels.
I hope this sheds light on this issue.
Marty from Kingston, Ontario, 
You are of course right on all your points. As it turns out, one of the big issues for me in this case was issue #1 - improper fastening. I laid the sheet down on a surfaced spoil board and clamped the piece down with sideways clamps (dog holes and tapered wedges). There was nothing holding the work piece vertically. This really came to bite me when I âsurfacedâ the second side where it was likely that relieving the stress on side one caused more warpage and the sheet just didnât sit flat. Side two required a lot more material removal than side one.
I have agonized over this issue a lot and I just canât see a solution. There are two potential solutions that popped into my mind - use a vacuum table (working on it) or use tape/hot glue etc to attach the work piece to the spoil board. Screws donât work unless you donât mind holes. As far as tape is concerned - once you surface the MDF it becomes fuzzy and nothing really sticks to it.
I have yet to figure out a solution to this issue 
@Jens FWIW, I have surfaced my spoilboard slats more than once over the years, and both double-sided tape and blue tape still stick to it just fine.
Thanks @gwilki, I will give this another try.
I have resurfaced my MDF spoil board twice.
After surfacing, I give it a quick sanding with 180 grit (with attached dust collector). I then roll on shellac. Mild sanding and another coat of shellac. Repeat 3x
This seals the mdf from moisture and gives a great surface for dbl-stick tape or Blue tape with CA glue.
Generally MDF is like a big sponge. The hard flat faces doesnât absorb moisture until you skin it. Until then the edges will usually be a bit fatter than the middle. If you skin one side and both the other without it being screwed down it will banana. If you seal one skinned side and not the other it will banana in the opposite direction. When fastening I find it best to pre-drill it because it will mound up above and below the fastener.
There is MDF rated for wet environments that should be significantly better since it is water resistant through the entire board. One is called Armorite and another one by Uniboard called NU Green. I actually can get the NU Green somewhat locally which was a surprise to me. The NU Green version doesnât have Zinc Borate added like the Armorite does which isnât necessary for this application.