I'm not trying to sell anything here, but I've done some of my own rust removal/paint work on my white Protege5 and have had decent results for very little money. I'm not skilled enough to cut out and weld new sheetmetal, and I'm not willing to invest in the car, so here's what I did for about $30:
1) Grind down the surface rust until bare metal is showing, either with an angle grinder or dremmel if it's a small spot.
2) I actually used a bondo body kit from Canadian Tire (about $15) when I found that the rust had actually perforated through the fender. There was nothing left to paint so I had to rebuild the surface. It was a simple process - mix the putty and the hardener, slap it on, shape it and let it dry. Sand by hand with a coarse grit sandpaper, then refine with a fine grit, and repeat the process untill the surface is rebuilt smoothly.
3) Last step is prime/paint. I used this product as a primer:http://www.canadiantire.ca/AST/browse/4/Auto/AutoRepairAccessories/SpecialtyRepair/PRD~0477956P/Rust+Check+Rust+Killer.jsp?locale=en
I covered up the wheels and windows and stood back a ways and took short, light sprays with the primer. After about 2-3 coats I used the Rust Check Gloss White paint from Canadian Tire, very similar in colour to my Mazda paint.
About 18 months later the paint is starting to bubble again, so it's not a long-term solution, but it's still white. For about $30 it looks a lot better than having big brown crumbly spots all over the car. Mine was on the doors, hood, and all 4 fenders.
Also, to remove the rust 'stains', use a product called Spray 9 and a kitchen cloth. It's a multi-use cleaner that costs about $4/bottle and it removes the brown stains on the paint very well. This was one of the worst parts about having a white car that is rusty - it stands out like a sore thumb.
Hope this helps!