The first one referenced by Janis (mkmini) says it is a phosphoric acid product. That would be excellent for rusty areas; phosphoric acid is a rust remover and barrier. It creates a chemical reaction with the rust leaving a phosphate barrier to protect the surface. The second product Janis referenced I can't tell from the label, but the fact it changes color ("more rust becomes more blue") indicates the same thing. The color change is typically the result of that chemical reaction. I don't recall seeing these type of products available in the US, but both of you are in Europe so things are different.
The product Ghostdancing referenced I believe is like "POR-15". POR-15 is essentially 'super-glue' (cyanoacrylate). I'm not certain, but I think that is what the PPG product is calling "nitroacrylic". It forms a hard layer to 'seal' the rust. Personally I am not fond of this approach to rust. At the chemical level, it is impossible to stop rust from forming by covering it up (which is what POR-15 claims it does). And it does not provide a good foundation for top coats (color coat) of paints.