Hi. I’ve got 1 like your red one, wetstone is usable, the “normal” 1 less so. The problem seems to be because the wetstone is driven through a worm-drive it is constantly overrunning the motor so the load is not constant, when using the wetstone the load on it keeps it from overrunning so is ok. I’ve relegated the other end to a wire-brush! I get into a position where I oftain have to clear old cruddy threads, perfect for that however rough it runs. Mine was even cheaper than yours someone gave it me when clearing out junk! Think it says “B&Q” on it.
Bob