Sand all to bare metal. Clean the surface. Apply Bondo generously. Sand rough. Sand smooth. Wet sand. Prime. Paint. Clear coat. Wet sand clear coat with a very fine paper to even out only if very noticeable. Fiberglass meshing should be used did there are gaps that need to be filled after removing the rust.

 

Here's a good video on how to repair that:

 

What I learned about trying prolong the life of this type of rust rot is: If moisture can hit the backside of the repair, bondo will fail sooner. Fiberglass will be 100% water proof and will stand on it's own as the moisture, rust, and rot envelops the area around the fiberglass.

The only true way to fix it almost permanent is to cut out all of the rotted metal and weld in new metal. Sorry, but that is the truth. Most don't like to accept that. However, a decent no-metal-replacement repair can last 2 or 3 years if done really carefully. Hope that makes sense.

 

Second that. Get a body shop to weld in a little piece of metal.
The only way.
Polyester is for carsellers with "guarantee untill the corner of the next street"

 

You have got a big hole in your fender. If you plan on fixing it with fiber glass, it won't last too long. Find a rust free fender. It will be better and cheeper in the long run unless you have your own mig welder and you can put a patch in after you cut out the rust. The way it looks, you probably can't, so find a fender.

 

The posts above are true.

My first car was a rust-bucket -- a 1960 Ford Falcon rescued from the back of my Uncle's property. Not an antique, just a 10 year old car that had sat there for half its life. Dad & I spend the summer fixing it up. Great bonding experience (ha-ha, I just noticed the pun).

Mine wasn't a perfect fix but I got a couple more years out of that car and the rust didn't show through but I knew full well it was on borrowed time. It did what I needed it to do -- got me through high school and college. So if you're planning on the long haul do it right. If you've got a window of 3-4 years then the fiberglass and bondo will probably get you by.

 

That is deep! make sure to also clean the sides even though the other still has black painting (we don't know if there is already inside).