My plans are to build some furniture so I need a really flat assembly bench. I built one out of MDF and surfaced planed it to perfectly level so I decided to build a second one out of sandply since I had the other flat one to build it on. I decided on a second plywood one because I was afraid that the MDF might not hold up. I tried to laminate it but it did not come out exactly level so I ripped off the laminate and surface planned it again with a router sled.
Much to my supprise the new sandply one is not perfectly flat even though I had it clamped to the MDF one at all times. Across the three foot width it is very flat but lengthwise is where I have the problem. About one third of the way from one end it has a slight dip of .020 of an inch that runs about eighteen inches or so. The .020 of course is at the center of that span. I would plane it as I did the MDF one but the outer layer of the sandply is probably no more that .020 and then I would run into some rough wood.
My questions are: Is this flat enough? If not, what would you do about it?