When buying rotary wire brushes its worth noting the difference between a normal type and the 'knotted wire' one. The latter are much more fierce, for heavier duty work, than the normal ones. If in doubt start with a normal one.
Screwfix sell a very nice 6" wire wheel that fits a 9" angle grinder, these are particularly good in that the wires are soft but the far greater control of the larger angle grinder make it far safer to use.
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