I have rust on my passenger side front fender, it is pretty bad. PO tried to repair it by just popping the bubble or what not, but since I've owned it, it's gotten worse, it is starting to chip the paint and burst open (kinda like a big pimple)
What should I do? Sand it, bondo, then paint just that part? I was thinking about replacing it but it's too much for me right now...
You can bondo it, paint it, whatever you want. BUT, GET ALL THAT RUST OUT! You'll need some super course sandpaper or a die grinder. I use my 90 degree angle grinder and grind 'er down until all of the rust pits are gone, completely. Then clean with alcohol or kerosene, something to get the dust off it. Prime it right away to prevent it from rusting again overnight. Then take your time with the finish work.
As you mention a pimple, if you don't get the root it will keep coming back out to the surface. Same goes for rust and you'd be surprised how fast. If you just cover what is there without grinding out the pits of rust, within weeks it will be back again...
I would highly recomend getting a new fender if you want to fix it properly.
Butepending on how much rust is actually there after you sand into it, it is possible to fix without the rust coming back. Like said above you would want to take and angle grinder to it and get it as much grinded off as you can, but depending on how much rust is there you could go through your metal and then you are bassicly screwed. But grind off all the rust and pitting. Wipe down with a wax and grease remover..not kerosene...apply a rust killer over the area where the pitting was. Let that sit for 24 hours, then lighly sand over it again and spray all the bare metal with a self etching primer. If you do not use selft etching primer, and just regular filler primer your repair will not last and you will have wasted your time and money.