yup, 'kurust' type treatment does nothing to any rust more substantial than that caused by a very new stonechip. i put some on a tiny patch of rust in my bootfloor, all it did was turn it a delightful shade of purple until i overcoated it, and then spread like rust would normally

grind it right back until its all gone, or cut out and weld, and then etch primer coat it, only way to be sure its gone