I wouldn't use a brush on the scales. There is a risk of damaging the scales further by pushing the crap that the SLM leave behind deeper under the scales, or in some cases I've seen, rip healthy scales off.
The scales have a type of hinge that allows for some movement as the toes and ankle bend. Older roosters tend to have old roosters legs and while the may look infected with SLM they are often perfectly normal.
The hinge is pretty delicate once the scales are raised.
It is worth mentioning that the raised scales will never return to their original position. The chicken has to shed the old scales and regrow new. This is the time when regular treatment (Vasaline, oil, whatever) can be most productive in protecting the new scales as they grow in.
A mix of betadine and vasaline (25% to 75%) is what I've found works well. The betadine is anti fungal as well as a disinfectant and will help clear away all the debris from around and beneath the scales. One needs to fully smother the affected areas in this paste; a light neat looking coating is not what should aim for.