you'll never succeed w/ either 4 OR 7" grinders let alone oscillating hand sanders,,, w/o pics, its a challenge trying to answer this but, IF you want uniformity of surface, you need to grind the whole surface,,, find a walk-behind grinder ( you can rent 1 w/110v capabilities ),,, start w/60 metal pucks then move up to 100's then on up til you're satisfied,,, btw, MUCH easier grinding wet !
the alternative is overlaying the whole floor w/polymer-modified cement,,, can't you find a throw rug or coffee table ? ? ? ? ? ?
i never had much luck w/quikcrete-type materials but we don't fool much w/lower-class quality stuff from apron/vest stores,,, currently i think were paying about $44 for a 50# bag of polymer-modified resurfacing stuff - white cement mix - including freight from indiana,,, what's the apron/vest stores' price ? ? ?