In response to the original questions regarding rust removal with: D-rust it, Safest rust remover, Evapo rust.
The most important, overlooked aspect of these fluids is that they are NON-HAZARDOUS, the 'greenest' of all rust removal technologies available today.
These products do not need to be neutralized after use due to their chelant based chemistry. You simply rinse the part thoroughly w/ water. One of the products mentioned will not leave any kind of coating on the metal after it is rinsed off. there will be nothing between the metal and the finish coat. In my opinion, this is the best adhesion one can achieve. The drawback is that the metal should be protected from further rusting with a spray on, 'dry to the touch', protectant or if small enough, wrapped in VCI paper, etc. The other two will leave a thin phosphate coating which will protect the metal from subsequent rust, but only for a short time. The coating they leave may, or may not be acceptable for finish coating.
These products will not cause any hydrogen embrittlement that I am aware of.
Other members familiar w/epoxy may be better able to answer the question concerning bare metal and epoxy.
If you refer to treating the inside with regards to rust removal, these products will remove rust as long as the surface can remain wet w/ the removal fluid, using either immersion or recirculation.
If you use these fluids in fuel tanks, clean them first w/ some detergent and rinse them out. These fluids will remove some oil dirt and grease, but doing it w/ the detergent is much less expensive. Let the rust remover remove rust, that is what it was designed for.
If there is rust scale, remove it first with a hammer, chain in fuel tanks, etc. Once again don't waste the fluid.
35 prog is correct in considering the continuous shower method. Once set up w/ the numerous , inexpensive off the shelf items, you let it do the work while you go to work, go to sleep, have a beer or whatever. I have used this method with much success in my repair shop.
These removal fluids rust removers are ideal for thin body parts. How many out there have sandblasted through a thin sheet metal part that otherwise would be intact if these rust removal fluids were used? They do not remove any metal unless left in the solution for an incredibly long time. Why would you want to leave them in longer once the rust is removed?
Crownver, You can de-rust an entire car if you scale up the continuous shower method. Plus you can spray it upside down to get under floors hardtops, trunklids, chassis, etc. The applications are merely limited to your imagination. Anyone can use duct tape, plastic, pump, hose and sprayer of choice. You can even use irrigation type pipe so that it drips or streams over the car to get the upper surfaces.
The black finish you refer to is not magnetite, but a soft black reside easily removed using a soft bristle brush (paint brush) and rinsing w/ water. That residue may fool you into thinking that the part is not even partially or completely de-rusted. You must check the object frequently, and clean off the residue to see the progress. If rust is still present(be sure to check the pits closely!) re-soak or shower again until removed.
35prog nailed it in his last reply. BTW: rotisseries work great!