Remove rust inside door panels?

I used this product...
I mixed it with 20% water like it says you can (in a smaller separate container)..
I went to the dollar store and got quart size throw away containers, throw away small paint brushes, and a couple of those dish washing wand with the little sponges going every direction on the end and used that to completely swamp out the insides of my doors and cowl of my truck....
And the entire bottom of the cab...