OK so I tackled one of the worst rust spots on my truck briefly. I used a 3M rust wheel thing - http://amzn.to/1NbUL5d - to grind off some of the paint and rust bubbles.

What do I do next to smooth the metal down further? I'm scared to grind because it will create sparks and it's right above the gas filler. If I disconnect the hoses going to the tank it's probably going to break off due to rust so I really don't want to do that.

Any ideas? I think I need to get the metal smooth then use a bit of plastic filler to smooth it over? Going over it with filler now will just cause it to build up higher.

Yes, I realize I'm not done grinding away at the edges, I just did it quick last night to see how well it will work.

I was thinking I could put a damp rag over the whole gas cap area and grind it outside where it is windy in case of any vapors / sparks.

Yea I would go with wet rage over the gas cap.
You could even get some paper and tape off the area where the gas door goes to prevent any sparks going in there.
First clean the gas cap area then tighten the cap and put a wet rag over it and tape off the entire little square then grind it smooth and fill it

Here's a quick picture I added some color to. The black color being tape over the square and later you can use a file to smooth down the edges of the square by hand to prevent sparks.

I guess my other question is what grit should I use next to grind on that? Worst case since it's a small bit I could use a sanding block with sandpaper and do it by hand to save on sparks.

Am I wanting 36 grit? I do not own a big electric grinder but I do have a small air powered grinder.

You could try the 36 grit.
I was using some 40 grit yesterday on a truck and got down to bare metal and seemed like the 40grit even then wasnt cutting the metal.

Try the 36 grit and if its not working for you, use a gridning disk on that area, not a cutting disk, the thicker grinding ones.
Try to hold it up to the panel as flat as you can and begin grinding off the rust. Dont worry about the gouges being left, youll fill it in later, but dont cut too deep into the panel. Just grind it till its flat and can be filled and sanded smooth.

That is what i would do at least.

If using a grinding wheel i would stay away from the edges and do those by hand with this.
http://drillsandcutters.com/10-mill-bas ... oCalfw_wcB

The 36 grit should cut it though with some time.

I just looked at the link of what you used to remove the paint, did you try that on the rust ?

What is your overall plan here? Is this a complete repaint or just a single spot that you want to address?
Looks like you have other dings and some rust in the picture as well.

On the area right above the filler opening, some 80 grit on a pneumatic pistol grip 3" sander will clean the pitting up well with minimal sparks. After the pitting is removed, block it with 180 and feather your edges. Give it 3 coats of primer then block it again with 220 and give it a skin coat of filler if nessasary; level your filler and Prime again and block it out and you'll be ready to go.

How about a spot blaster. Fairly cheap and some play sand can go a long way. Plus if you have other areas you can still use it on them. It's just a thought.

Ripping off good metal with 36 or 40 grit is not the proper method. Tickling with 80 grit maximum is sufficient. All you want to do is remove the surface rust and scale in the pits. You do not want to take down good metal to the valley of the pits. Using a spot blaster like Mike said is the easiest way. If the pits are deep, you may go right through after prepping. That 3M wheel will remove the majority of the rust but not all. Get the panel real hot and it will warp. And when metal rusts like that is swells, so the area may now be high. Remove all the rust by mechanical means. Phosphoric acid removes rust also but must be completely neutralized prior to primer.
When 100% of the rust is gone, skim it with body filler to fill any deep pits. Or epoxy the entire truck after prepping and then do your filler work.

By that picture you havnt found good clean metal yet surrounding the bad area.
What is being said by Jim, Scott, Mike is all true and good advise.

keep cleaning paint off though till you find rust free metal then blast clean with Glass bead then hit it with phosphoric acid and clean again with soap and water cover it in epoxy or better yet Acid etch primer then regular primer then use filler if needed prime and paint.

NO Guaranty it wont come back though only sure way is to cut it out and put in new metal.

I bought this spot blaster cause it was on sale like a year or two ago, have not used it yet - http://www.princessauto.com/en/detail/s ... -p6320642e

I am concerned though about static electricity right there so hesitant to use it right by the gas filler.

Overall plan is to use the truck to learn bodywork DIY. Total newbie to most of this stuff. Every body panel on the truck has a dent or rust, so I'm just going to use it to learn on. I don't know if I'll ever paint the whole truck but if I can at least get some skills grinding and using bondo, then primer it over top for now I'm good. Probably would like to spray a whole panel with an HVLP gun at some point but for now I just want to fix up the individual spots.

Whole truck view. Behind both rear wheels is pretty rusty but no holes so should be able to save those. The rockers & cab corners have holes so that's the biggest things to tackle (starting with the smaller stuff). You can't even buy new rockers & cab corners and that's beyond my ability today, so I'm going to chip away at it all, probably one panel at a time until I can get a game plan together on cutting and replacing metal (that's a whole new topic LOL).

The truck looks a lot better in these pics than it actually is. There are endless paint scratches everywhere and many, many dents. The hood appears to have a few extra coats of clear coat on it, as it chipped in a couple places. Everything but the hood appears to be factory original paint.