Rather than using a big gob of "bondo" to make this repair,
let's repair it properly by using all new metal.
Kaiser Bill's Fender Repair "Kit" is designed to allow you to repair an extensive
rust-out as shown in the top photo of the passenger's side
or a less severe rust-out as shown in this lower photo of the driver's side.
These instructions will lead you step-by-step through the repair process of both.
Disclaimer: Since each person's skills are different and they totally control the tools, supplies and processes used in the installation of these parts, "Kaiser Bill" Brown does not guarantee or warrant the parts or instructions furnished with this kit will perform to your satisfaction.