I have some rust on my prius. There are some areas of pretty extensive rust like behind the wheel (image 1). There other areas there are small spots of quite superficial rust (image 2). Finally, the are some areas where there is just no paint (image 4). I am not hoping to get it done professionally as it might be expensive. Looking for a cheap solution even if it is not cosmetically pleasing. Someone suggested using sandpaper and then finding any ordinary paint.
I investigated and used some rust converter on the undercarriage of my 1985 Toyota 4wd. There are a few different types but most use tannic acid to convert the iron oxide into ferric tannate, which is much less prone to oxidation. I've found this to be more effective than sanding and painting, as it's very difficult to physically remove all the oxidation and it will often continue to oxidize under the paint touch-up. The most effective ones go on reddish or silverish and then turn to a dull black hue as they react. They do quite a good job of stabilizing the oxidation and protecting the surface. They can then be painted over to color match.
You need to get most of the rust off then use the converter, then bondo, then sand, prime and paint. The paint can come from Toyota in a spray can.
Usually, rust near the wheel well, as in the OP's photo #1, starts from the inner surface of the fender. So any cosmetic treatment will be short-lived.
I've seen that on Fiats and early Datsuns. 3M Rust Reformer and POR15 paints work very well on chassis and bodywork.
Thanks. The car used to be in Michigan and saw a lot of salt. The car was worth 5k about 5 years ago. I am thinking it is just not worth getting professional body work on it. So, may be I will try to sand I away and then use the converter and then paint over it. Will let you know how it goes.
Over the past 50 years, I've tried everything on rust. I prefer phosphoric acid, which is the choice of must industrial production fabricators for "pickling" iron and steel for plating and plating. One brand, Krud Kutter "Must for Rust," according to their MSDS is 47% pure phosphoric acid. Other brands like Naval Jelly contains many other aggressive chemical agents that enhance it's action, but leave a residue. Phosphoric acid, when left exposed after treating, continues to convert and deep seam rust into Iron Phosphate, which is dark gray and seals rust. You can leave it on for a month, wash it off and let it dry and then coat with an auto rustproofing material or paint and primer. I've found this method to be most effective for rusted metal.
