Fixing a rust hole on the rocker panel can be expensive. I've done body work on cars for a lot of years, and have developed some techniques to make the job fast, easy and cheap. If you're trying to get a car through inspection and you want to car to look acceptable there are some shortcuts that are effective. Most rust on a car happens on the rocker panels. I use structural foam to fill the holes and then coat them with Body filler to give it some nice structure. Then I paint the bottom black so there is no finish work. The entire process takes about an hour. And it only cost about $20.
Step 1: Fill the Hole
To fill the hole use spray foam and fill it is full.
Step 2: Prep for Bondo
Once the foam firms up, cut if off with a saw and then sand with a rough file.
Step 3: Bondo
Put on the Bondo and then put a trash bag over the Bondo so you can shape it under control. When the bondo drys pull the bag right off.
Step 4: Paint!! :)
Put tape where you don't want paint, and then spray paint the panel. When the pant drys take off the tape.
Step 5: Finished
Now take a step back and take at the work you just did. This is fix to get you though inspection in about 45 minutes! I wouldn't do this to a show car, or to a car you are trying to flip and sell to another buyer and not tell them. This is a fix to save money and get passed inspection. By doing this you are saving $500 by not spending $500 dollars on a new panel!