Welcome to the forum , Acamar .
I'd go along with waywards' idea of pressure-washing , to clean your item .
I'd use some soap & steelwool too .
I'd guess that this was a 'working unit' in the dairy 'field' , so I'll say that you might find some wear to it's original finish , under the paint .
Usually , cannisters of this type were scrubbed with big ole scrub-brushes .
Since you'd like to use this item for umbrellas , perhaps coating it with something like an acrylic-based polyurethane clear finish would work well .... after all , if it will be sitting on your floor , the rusty surfaces may eventually damage the finish on wood/carpet/flooring sitting below it (light-colored umbrellas too) .
These little guys can be also used for some folk-art/deco painting , if you get the hankerin' to do some .
That's my 2 cents .