Rust issue
 

My 2000 Mustang V6 is starting to show some rust above rear tire on driver's side.

Any suggestions on how to postpone even more rust?

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How's it look inside the fender? If it's only as bad as that, fixing it permanently involves sanding the affected area down to bare metal and repainting. I know that sounds bad, but with rust you want to fix it right, as there really isn't a good "temporary" fix for it and it'll only get worse.
I had to redo the whole bed of my truck a couple years ago because it looked that way.

From the pics they look like bubbles, when you grind them you are going to get holes. filler really isn't the best way to fix it at this point and the problem will come back rather fast for sure. when someone has a spot like that the best way to fix it is to cut out the spot and make a patch to weld in, skim coat of bondo, prep the area and blend. If the holes are small enough and you grind all the rust around the area out and get it clean you fill the holes with weld if you are good enough with a welder and careful not to blow more holes in the metal and only of the holes are small, this can be another option.

If it wasn't going to make holes then some rust converters would be an option after grinding out the rust but with holes patching is the best solution imo and I has to do the same thing on two very small area of my stang during my resto to make for sure the rust was gone, cut out the cancer.

Crap ... I was hoping it wasn't that serious. Thank you for your help guys!

Yeah sorry. you have the bubbles, bubbles = holes. Best of luck, but it isn't that bad.

Dude, you're like Car Jesus. Between you and Kevin509, there is a wealth of info. Seriously, thank you both for your input on all thing car care and potential fixes to problems.

The rust is on the inside. That looks like the part that is accessible from the trunk. You'll need to fix it from the inside first. Also find out why that area is getting wet.

If it's the front fender it's much easier. Remove the fender, clean out and debris, and fix it from the inside. Make sure the splash guard etc is in good working order.

If fixed from the inside you can get away with doing minimal work on the outside. But ideally it should be sanded down, rust removed, painted.

Thanks. I always like helping out if I can .