Can you shim it level?

You might try a router bridge as shown in this article.

Definitely use a router bridge!

I just connected two pieces of angle with two scraps of wood and some screws.  The two pieces of angle need to be parallel and roughly the size of your router base.  The wood pieces should be attached so that they just clear the width of the bench.  Then clamp a straight board to each side of the bench.  Make sure the boards are level and that the router bridge is level when you place the scraps connecting the angles on the side boards.  The bridge needs to ride just above the bench and not touch.

Put the widest straight bit you have in the router and set the depth to just kiss the lowest part of the bench.  Then just run the router across the width of the bench, move the bridge and repeat until done.

You may need to clean up router marks with a light pass of a sander.

The bridge I made has been dissembled and the parts reused, so no pic of that, but here's the bench....

each slab was milled perfectly flat separately and then joined.  I'm not sure how they can sag as shown in this picture.


You can see the endboard being flush with the top surface of the two slabs.  It was straight when I mounted it but must have bowed in the interim. 

 ...The top is held on by essentially a M/T joint, really more of a 1 inch peg into a hole.


I have the end pegged so there is no way to remove the end board.

It seems to be resting flat on the base but isn't flat on top.     Strange

I did the router bridge operation on a benchtop consisting of three glued layers of MDF a couple of years ago. When done it was dead flat, and within a week it started to bow up in the middle. Both top and bottom surfaces were sealed. (We had a long discussion of this on the previous Festool Owners Group.)  We didn't come up with any authoritative conclusions at that time. Maybe there were some built in stresses during the glue-up of those three layers. I'll never know.

I am glad to say, however, that the top of my MFT (only a single layer of MDF) is still dead flat.

Charles

The top may be resting flat on the base but I think the laminated base is not flat itself. Have you checked it?

Howard,

I live in NE Ohio; the humidity level where I live is generally high, but swings fairly widely between its low in winter and highs during the other seasons.  I strongly agree with Eli's recommendation that you should apply equal layers of sealant to all surfaces, but even then the bench top may not be stable and remain flat when subjected to humidity swings.  I note that you live in an area of Texas which I presume has fairly low humidity at this time of the year (January - Winter).  Was your bench top flat when you first assembled it?  Does it become more or less crowned with seasonal changes in humidity?  From your photos, it appears your underlying cross supports are made of laminated 2X4 stock.  Normally, such construction grade lumber is not dried to as low a humidity level as are hardwoods used in furniture (and workbench) construction.  Thus, I would also be suspicious of the flatness of those laminated cross supports, and recommend that you confirm they have remained flat and straight.

Here's another tip/trick you might try - letting nature work for you to straighten out your top, although I have never tried this on such a thick workpiece.  When I have needed to straighten (bring back to flat) a glued up panel that warped after glueup (due to sapwood and different shrinkage rates), a trick I have found very effective is to lay the concave side of the panel down directly in contact with the grass of my lawn on a sunny, dry day.  This creates a moisture differential between the underside facing the ground and the topside facing the sun.  Using this technique, a 3/4 inch thick glued up panel of black walnut with a 1/2" crown straightened itself out in about 1 - 2 hours, and I assembled it into the rest of the case box quickly before it warped again.  (I still have that stereo cabinet and that end panel is still flat and did not crack!)  A thicker workpiece would be expected to take longer, but it, too, should expand more on the side that is facing the soil, and thus cause the workbench top to become flatter.  But then you would have to reattach it to sturdy and flat support membes before it tries to rewarp, shimming it as needed when fastening it to your cross-members to bring it flat.  And seal it - all surfaces.  Note that shellac is an excellent sealer and surprisingly has lower permeability to moisture than most polyurethane varnishes, despite the fact that shellac is attacked by moisture.  I seal first with shellac, then overcoat with varnish or lacquer.   I have a 30 year old workbench (that I purchased complete) whose ~3 inch thick top is formed of glued up strips of hard maple, with 3 embedded steel reinforcing rods.  Despite all surfaces being sealed, and the ends further sealed with parafin, it has become crowned about 1/8 inch with the center being high, and the amount of crown changes with the seasonal humidity change.  I just live with it, and much appreciate my Festool MFT top that is relatively very flat.  I use the MFT top for assembly to keep drawers flat during glue-up.  I just finished making 5 drawers that are 24" deep by 33" wide and they appear to be dead flat.  I use the maple workbench for heavy duty work, for planing and pounding, including automotive repair work.  Good luck!!

Dave R.

I've used a finishing system similar to what you describe for some laminated maple tops, the key differences being that I used Watco (Danish type) oil on ALL surfaces to seal them before I top-coated with solvent based polyurethane.  Of course, I let the oil finish dry a considerable length of time (weeks) before applying the polyurethane.  I'd try to seal all surfaces, and recheck your base supports.

Dave R.