Are you having trouble with badly out of flat backs on newly purchased gear? I must be lucky. Most of the old stuff I bought was flat and the new most certainly was - LN and Lee Valley.
It's a mixture of LN, LV, Narex, Stanley, Buck, Witherby, and some others, but that's not the problem. The problem is that my early (clumsy) attempts to flatten the backs actually created the problems that I am now trying to fix. My first mistake was trying to use powered sharpeners (Veritas Mk II, Viel 1 x 4 belt sander, and a Bosch 3" belt sander) before I really understood what I was doing. In an attempt to fix that, I switched to PSA sandpaper on granite, which worked, but led me to discover the concept of dubbing and the cost of 3M Gold Stickit rolls.
My hero, Stu Tierney (Schtoo), has come to the rescue. My newly acquired collection of Sigma Ceramic stones (120, 400, 1200, 6000, & 13000), a pair of Atoma plates (140 & 400), and some Silicone Carbide grit seems to have solved my problem. In all fairness the 120 and Atoma 140 just arrived yesterday. As a bonus, however, my edges are beyond sharp. I know the 13000 is an overkill, but I can't resist. It is a bunch of work, but I have now successfully re-flattened the backs of all of my Lie-Nielsen and Narex chisels and have started in on the vintage ones. I have also re-flattened a few LV and LN spokeshave blades and a couple of LN plane blades. Still a ways to go, but it feels good to know that I can get to flat.