I got the impression that the filling patchwork had been done. If so, & if your patch material is well proud of the surface, a belt sander can save you hours of labor in bringing it down close to flush, then finish by hand sanding with finer grit papers.
The above posts correctly point out that proper filling to begin with will greatly minimize later shaping/sanding & also eliminate the threat of material hungry power tools. I especially like Jeffís suggestion of troweling in the first layer, shaping ridges, then using them to depth
gauge the next/final fill. I'm good at this kind of stuff & am myself adding that to the bag as a new trick (thanks Jeff).
Other examples (I've been through many houses in the last several years) I've gotten to where I simply don't sand drywall spackle. I apply it carefully to begin with & use moist terry cloth hand towels to achieve a final surface prepped for primer & paint
Similar thing with auto body putty (I personally have avoided fiberglass
& haven't even used it on cars, I'd rather cut & weld & use steel
where others would use 'glass). With putty I shape it carefully to begin with, then use a selection of the above described perforated plane(s) to establish the surface as the bondo hardens. By the time it's hard enough to resist hand working to any degree the surface is there & requires only spot filling & detail sanding, not heavy stock removal
Similar thing with epoxy
wood fillers, etc, etc. The idea is to get it as close as possible to it's final shape while still readily workable or, as Jeff suggests, reduce the surface area to be shaped, then fill final.
It's easy to get impatient & slop in filler materials. Further, the temptation to lay it in well proud of the surface (just in case) can be almost irresistible. Youíll find with experience though, that itís far easier to fill with restraint, then spot fill minor voids & hollows than it is to remove heavy material. Putting your time & effort into the filling phase of the project
is well worth it in saving time, material, effort & mess.
If, however, you're already past that point it's hard to beat a high rpm
belt sander with a course belt for stock removal
. Avoid long grinds on one particular spot, the heat build up will be fast but in any reasonable situation long grinds just arenít necessary, the material that you need gone will be dust in no time flat.
As with anything of this nature, if you're unfamiliar with a given process & the tools/materials required, itís always best to first get in some practice on something that you don't care about. First time use of a belt sander can be comedic, instructive & (or) highly destructive. Avoiding the hull
of your boat until you've ground down sacrificial stuff & have a feel for the tool can help to keep that first touch-off in the comedic & instructive realm where it belongs.
If you get really good with patching & filling in general youíll find that you have no need for the violence of a belt sander. Or maybe youíll get really good with the sander while you master efficient filling & end up using it for things like rapidly scribing in cabinet ears (match line to an imperfect wall surface) like I do.