There are a few options here as far as de-rusting your tank. My 175 tank was horribly rusted, so I did a 2 step technique.
First off, I dropped a bunch of screws, nuts, bolts, pretty much anything metal with sharp edges into the tank. Taped it up, wrapped it in a blanket, shoved it in a 5 gallon bucket, and rolled it back and forth across my lawn with my kids. Others do the same thing but put it in their clothes drier on fluff mode. My wife wasn't okay with that, so we did it the bucket way. Once you feel it's done, take the tape off and dump out the rust powder, along with the stuff you put inside. Make sure you don't have the petcock installed when you do this.
After that, I still had a little bit of rust inside. I sealed it up again with tape, and filled it to the top with apple cider vinegar. I then let it sit outside for a few days, and after 3 days or so, there was no more rust inside at all. The vinegar attacks it and removes it from the metal. After you drain the vinegar, you'll want to neutralize the reaction with some baking soda and water to prevent flash rusting. I then sprayed some WD40 inside to get out the rest of the water, and finally gave it a few thorough rinses with fresh gas.
You can follow this up with a sealing coating, just don't use Kreem. It's the worst. I've heard good things about POR15 and Caswell systems.