my 2 cents on the Bondo thing.
You are not so much covering the entire tank in Bondo as you are skim coating it. I say skim coat because you are not putting it on thick. A very thin layer and then block sand it with a flexible block. NEVER USE YOUR FINGERS WITHOUT A RUBBER BLOCK.
Once you sand it down there will only be a few tiny spots that still have any Bondo in them. They are your low spots. Circle them with a sharpie and then skim coat it again. Then sand it again. As you sand away the bondo the sharpie circle will show up. Do you have any spots without circles? Those are the ones that you missed with your first attempt.
Primer; A filler or etching primer works good for me. Lay on a few coats and wet sand it smooth like glass. Use a squeegee to look for low spots. They will be darker than the rest of the tank. If you have any small low spots you will need to prime and sand again.
All of this takes time. Days in fact. The end result will be worth it.
The more time you spend on your paint, the more time you will spend enjoying it.
