What he said about the resins/coatings: never tried them (never needed to), but I've heard plenty of the same stories of guys saying how great they were when they first tried them, and then finding out later on that the blocked fuel lines or crapped-up carb problems they were having down the line were the doings of flaking/dissolving coatings. Too aggravating and unnecessary to even try, AFAIK.
Here's the Lambretta tank I mentioned earlier. It was worse when I started, but this is the best I could get with Coca Cola (contains phosphoric acid) and a chemical drain cleaner with coins:
This is the point I decided to go ahead and put those Chemistry lessons to use.
BTW, I mentioned phosphoric acid earlier (which also works great), but in the example below I actually used Hydrochloric acid (what I had available at the time). This could have been done several ways, but here's the process I followed in this case:
1. Make sure to seal the bottom of the tank (so it doesn't leak on your feet), and WEAR GLOVES! (not the thin latex type; the thick kind used to manipulate acids). And keep a bucket of water nearby in case you get some acid on your skin somehow, to plunge the body part affected, or splash yourself if needed.
2. Pour 1 liter of water first into the tank, then slowly add a liter of hydrochloric acid. Golden rule: always pour water first, THEN acid; or in other words: pour acid into water, NEVER water into acid. The mix will seem to boil and fume like hell (the cold water and acid really heat up fast together), so avoid breathing the smoke. Swish it around for 10 minutes (it started appearing clean before that).
3. Pour the acid solution out of the tank, and rinse with water 3 times.
4. Pour 1 liter water into the tank, and add 1 liter lye (sodium hydroxide). Move the solution around for 5 minutes. (This is to reestablish a neutral pH, but rinsing plenty should achieve the same thing).
5. Again, rinse thoroughly.
6. Dry with a hair drier.
Tip: I never tried this at home because of the toxic fumes, but you should be able to safely pour the HCl in a toilet bowl (it will clean it up nicely), and then the NaOH when you're done neutralizing your tank. That basically turns otherwise separately aggressive chemicals into H2O (water) and NaCl (table salt).
Result: