I am using a cotton swab and it seems to work well.
Wide brush works fine for me. Every new rusting round I change the direction of applaing rusting solution by brush, from muzzle to the breech and vice versa. Also every new time I try to relocate the lines, where little overlaping occures. Just it needs to remember do not apply solution second time on the same spot, moreover never rub the same spot by brush or swab, whatever. Swap has to be allmost clean after applaing, but if too much overlaping occured swab will be dirty.
