If your car has developed a rust hole in the fender, some people might tell you to get the entire fender replaced. That will require you to find a body specialist. They will charge you for the new fender, as well the labor involved in removing the old one and installing a new one. A cheaper alternative is to swap out your fender with a used one. The problem with this is that the colors might not match exactly, plus it can be difficult to source the fender. You might spend many weekends hunting junkyards for a suitable fender.
The alternative is to fix the rust. It requires a bit of time, and some special tools, but if you want to preserve your car's body, and save money, then this is the way to do it.
First Thing: Grind Out The Rust
The first thing to do is get rid of the rust. You don't want to leave any rust behind. Think of it like a dental cavity. You need to get in their with a drill (or grinding saw, in this case) and get rid of the entire thing. Rust will spread, so it's vital that you remove all rust.
Use a small, electrically powered grinder. If you don't own a grinder, you should consider renting one or buying one.
Grind down the rust until you see bare metal. Don't worry about how it looks now, the important part is to make sure you get rid of all of the rust.
Next Step: Chemical Protection
You might have missed a bit of the rust with your grinder, but don't worry, you can take care of it. Purchase a rust sealant spray. Apply it to the bare metal. It will either be a spray, or in a paint can. Either one will work for this application. The purpose is to take care of any small, microscopic bits of rust you failed to remove. Also, it will safeguard against future rust.
Fill In The Empty Space
Next, purchase a can of auto-body filler. This will be applied like joint compound. You can use a wire screen patch to "hold" the fill. Place the screen over the hole. Then spread the filling material over the screen. Let it dry according to the directions on the can.
Sand Down The Fill
At this point, you need to sand down the fill to make sure it's even with the rest of the fender. Choose a medium grain sandpaper, something along a 120 grit.
You can paint a small patch on your car with a brush. The ideal way to apply auto body paint is with a spray hose, but a brush will work fine for small areas. First get an auto body primer. Apply this and let it dry. Then find the color that matches your car. You can order this from your local dealership. Apply this paint. If there are any bubbles in the dried paint, you can remove it with a very fine sandpaper (240 grit or above.)
For rust sealant, contact a company such as ValuGard.Share