When they say it doesn't damage paints and powder coatings, they mean it. Make sure that if you want to get to rust residing underneath it, you remove the paint/coating first. On the positive side, as you can see in the photo of the painted door and the backing plate in the above links, it really doesn't damage either. All of the other information I've found and uses I've tried have shown it to be completely benign. Just plan on it taking a while if you use the Gel to do it.
When applying the Gel to vertical surfaces, make sure it is applied at the proper thickness and with sufficient time allowed between coats to keep it from running, but not so long that it dries. If more product is applied to still-runny Gel, it will just take the already applied Gel along with it. If the Gel is allowed to dry, it will act as barrier between the subsequent coatings and the rust.
Bristled paint brushes work best for applying thinner coatings of Gel (as opposed to foam which don't carry much and have a a harder time releasing it when applied to the surface), especially when multiple thin coats are used. Using a nylon detail brush to daub on the Gel works best for applying thicker coats. The nylon bristled detail brush with it's wider gaps between clumps of bristles allows it to carry more and also makes it easier to release larger quantities in a controlled manner when daubing than a paint brush. Foam paint brushes and to a lesser extent bristled paint brushes make it difficult to apply thick coatings of the Gel because they are only capable of carrying so much at a time and work like squeegees when attempting to apply extra coats to already wet coatings.
Spraying the Liquid on the rust and allowing it to soak in (the rust will start to darken thoroughly when this happens) before applying the Gel seems to aid in the Gel's ability to remove rust. Especially thick, porous rust. Wipe off the excess Liquid or allow it to dry until damp on its own before applying the Gel, or you will end up with a soupy mess.
If you want to remove rust more quickly than just applying the Rusteco Gel and rinsing it off when it finishes working, then more aggressive removal methods are in order. The use of a steel bristle brush seems to be the most aggressive, while using a brass bristle brush, scouring pad or steel wool will be less aggressive on the surface but also less effective at removing rust. Use of a soft bristled brush such as a nylon detail brush or toothbrush, or scrubbing with a soft towel/rag/cloth will help aid in removing rust while helping to preserve more sensitive base materials such as paint, plastic, carpet, etc.
There are basically four different approaches to using the Gel without using expensive equipment. The first and most benign method (as well as the least aggressive rust remover) is to apply it to a clean part in two or three steps depending on the orientation of the surface (vertical or horizontal). For vertical surfaces, I found first working in a thin coat with a bristled paint brush, followed by at least two subsequent thicker, but still not runny coats (again, applied with a bristled paint brush) works best. For horizontal surfaces, apply it by first working a thin coat into the rust with a bristled paint brush, followed by a very thick coat applied with a nylon detail brush. In both case, spraying the area with Liquid and allowing it to soak in (then removing the excess with a paper towel or allowing it to dry until it is damp on its own) before applying the Gel is more effective on rust then just the Gel alone. After it has the opportunity to work (turn black) remove it with a toothbrush or nylon bristled detail brush or by rubbing with a paper towel or cloth (all utilizing water).
If the above method is too slow for you and the surface you are working isn't effected by more abrasive tools, using a steel or brass bristled brush (or scouring pad) will help speed up the rust removal process but will also negate the benign characteristics of Rusteco
The most effective process I've used to remove rust utilizing the Gel works like this: Spray the clean, dry surface with Rusteco Liquid, let it soak in and scrub with a steel bristle brush. Wipe excess Liquid dry, then apply a thin layer of the Gel with a soft bristled paint brush, working the Gel into the rust. Give the Gel sufficient time to work (darken the rust again or you can wait until the Gel starts to turn green, but don't let it dry) then, again, scrub with a steel bristled brush. You can choose to remove the excess rust permeated Gel or simply move on to the next step. On a flat, horizontal surfaces, follow up by applying another coating of the Gel (if you didn't wipe off the previous layer of the Gel, try daubing on a thin coat of the Gel with a nylon bristled detail brush as it will help to keep from contaminating your container of clean Gel with rust that will ruin it and will also help to apply another layer of Gel over an existing layer rather than just wiping off the existing layer as the already present Gel will work to keep the new Gel from transferring to the substrate) and give it time to work then scrub with the brush and remove excess if desired. Repeat as many times as you wish, but if you need to stop for an extended period of time (enough time for it to dry) than apply an extra thick coating with the nylon bristled detail brush and leave it to work, scrubbing off the hardened Gel later with a brush and water. On vertical surfaces apply a thin coat with the bristled painting brush, give it time to work, scrub, (wipe if desired) re-coat and repeat. If you need to leave it alone for sufficient time to allow it to dry, apply three thin coats as mentioned above.
If you would like a method that is more effective than the first method described but not as abrasive as the last two, than the last method will also work (although far less effectively) if you use a soft bristled brush to work it between coatings and for removing a dried coating should you need to leave it for extended periods of time
For mild surface rust, it seems applying the Gel straight without first spraying it with the Liquid is just as effective as spraying it
Softer corrosion such as aluminum and brass seems to come off very easily and quickly by just scrubbing with a wet towel/cloth/rag, toothbrush or nylon bristled detail brush after applying the Gel without any surface prep other than a thorough cleaning
I suggest trying the Gel in a small area to test to see how quickly and effectively it will work in your application to see which methods will be needed for your project
As I mentioned above in the links, if you have a heavily rusted part (especially porous cast iron), you will probably find it more cost effective and definitely more efficient if you purchase enough Liquid along with a tub of sufficient size to dip the part rather than to try cleaning with the Gel. However, if you can't dip it, the Gel is the way to go.
Once it goes black, it appears that the Gel's usefulness at removing rust is done