I notice that my MM252 seems a bit smoother than the (inverter) mm211 - not huge, but if I'm not "offroading" (the welder, not the sport) I tend to grab the 252.
I have one of the "greenies", the Power iTig200T - still haven't used the TIG side yet, but the arc beats my old 250 amp ac/dc stick machine, maybe not in longevity (too soon to tell) - I think a bit of hot start and dig is the difference, the old copper wound tranny machine doesn't have it.
Glaze, mill scale - SUCK. If cuts can't be done on the band saw or dry cut, I tend to reach for a grinder/zip disk rather than the plaz (Hypertherm PM45) - gotta use a grinder to knock the glaze off anyways.
Mill scale - tried HCL, doesn't do that well and can't breathe around it.
Tried sand blast, kinda like tryin' to push a wet noodle uphill on a gravel road with your nose (and no knee pads)
Depending on how smooth I want stuff, a lot of times I'll attack the mill scale with a thin cutoof disk, light pressure and continuous movement, then when I get thru in most places I'll grab another grinder with a 40 grit flap disk to smooth it out - seems to lessen the monkey motion some.
Most of the square tube I get (except heavier wall) is P&O, if I'm doing a bunch of that I'll lay it out on a pair of sawhorses, wet it down with degreaser and use a stiff brush floor polisher (electric) all over, flip all tubes and repeat for each side, then hose it down. Sunlight helps to dry, otherwise I'll warm it up with a torch/buff/weld, or fire up the backpack blower and dry it with that. Once that's done, I better be ready to paint THAT DAY...
For painting I'll usually just use a 6" fine wire brush on a high speed drill to give it some "tooth", then rattle can with a fan spray nozzle; haven't gotten into powder coat yet, but DID score a free lab oven off CL last year - temp controller's bad, elements are fine - 35 years in industrial automation/control, guess how much of a prob THAT's gonna be to me when I finally get that "tuit"
I don't think there's an easy way to do ANY of the prep part, but I keep looking for easy(ER) - clearly stickin' to that definition of INSANITY

... Steve