Heavily Rusted Cast Iron
Rusted Brake Drums
I used quite a bit of trial and error on these drums before I finally figured out how to best utilize the gel on thick, porous rust that had deeply permeated cast iron. Click image to enlarge.
Brake Drum in Gel
At first I tried just putting it on in a few thin coats, letting it turn black and then washing it off. I used that technique on both drums but I wasn't seeing much in the way of results. Then I tried a new approach on the drum in the photo above, where I applied an extra thick coating of gel and waited for it to turn black, then scrubbed it off with water and a wire brush. This worked much better, but didn't remove all the rust in one application either. Finally, I got tricky and decided to spray the drum first with the liquid, let it soak in, and then applied a thick coat of the gel. This worked much better, but I still wasn't penetrating the rust fully in one application, just working away at it. Click image to enlarge.
I finally decided to try a different method on the other drum (left image) and first sprayed it with the liquid, let it soak in and scrubbed it before working in a thin coating of the gel and scrubbing that. Then to finish it off I applied a thick coating of the gel, letting it start to change and scrubbing it when I had the opportunity, applying more gel when needed for coverage or washing it off with a wire brush and water when it got hard (such as overnight when I just didn't have enough time to dedicate to the rather labor intensive process). That worked the best cutting the de-rusting time with the gel as compared to the thick gel coating by 2/3rds (that is applied over the spray-soaked rust and removed with a wire brush and water after it has dried).
All told, it took approximate 15 coats of gel to do it the "thick coat" method (right image) versus five coats for the more labor intensive "apply/scrub repeat" method. I would definitely use the liquid on deeply rusted cast iron (see the rusted rear end weight above in the liquid section for more information) for both speed and cost, but if you can't dip the part and you can't sand blast it, I don't know of a better way to remove this kind of nasty rust. Click image to enlarge.