Well after futzing around with my dust shoe and a few other distractions, I finally got around to flattening my top... Here's a video I shot of the dry run and the first pass ...
I built the router mount with some intentional play for this very thing. I knew it'd have to be done and was prepared. I'm kinda surprised it was as close as it was, actually. The mount has slightly oversized holes which allow me to wiggle it around a bit. I still need to play a little bit with the same adjustment in the X plane but I don't think it's too bad as I ran a couple cross passes earlier that didn't seem to have any trouble at all.
Anywho - after a .005" second pass, all the pencil marks were gone which was nice - i figure i was out only .010" total across the entire surface. Not bad for an MDF torsion box without actually TRYING to get it very flat!
With it now flat, I've begun the fun task of coating it with poly. My reasoning is that MDF is not uniform in density - the top surface is much denser than the inner stuff and machining off that top "skin" has exposed a much less durable surface. By thinning down poly and flooding it on till it quits soaking in, my hope is that this surface will be very durable and should accept double stick tape really well.
I first put a 15-20% mixture of poly on, it soaked WAY in - used a whole pint jar on the first pass. The 2nd coat was a bit thicker, about 30% and soaked in another pint jar's worth. The 3rd pass today was about 50% and I had to work to get it to soak in all of that pint jar. I'll let this dry a couple days then lay down a full thickness coat and probably run a very light skimming cut to knock down any swelling - probably another couple coats of poly after that and it should be a finished surface that I can use for years.
Thanks for lookin!