It's possible to measure the thickness of metal with ultrasound. Since water is heavier than oil, but has water vapor, you only need to check the bottom and top of the tank.
Yes, the wall paint is useless and in the way of proper paint prep. Despite what certain TV ads say, you can't really paint over rust.
The ideal coating is media blast to near white metal, then MIL-PRF-26915 primer, epoxy primer which can accommodate a variety of topcoats, then a 2-part LPU coating. We can whittle that down to something achievable.
I have had very good luck with the following:
- Create very dry conditions, long enough to thoroughly remove moisture from rusty areas, i.e. Park a dehumidifier there for a couple of days.
- Wire brush, sand etc. down as clean as possible. Any paint stripper should easily take the latex paint off.
- 2-cloth wipedown with common paint thinner to remove contaminants
- Rustoleum rusty metal primer - though cheap, it performs surprisingly well over that last little bit of rust you can't get off without blasting. Stir the can very thoroughly, the sediments at the bottom are your friend.
- couple days to dry; quick swiff-sand with a scotchbrite pad to remove gloss
- rustoleum is not a final topcoat. Overcoat with ideally a decent 2-part epoxy primer or paint.
If you don't use a 2-part epoxy, then finish it up with an alkyd (oil) topcoat.
If you do use a 2-part epoxy primer, you're done if the tank is shielded from the sun. UV light from sunlight will wreck epoxy. So you'll need to swiff-sand again and topcoat with, well, you can topcoat epoxy with pretty much anything, even that wall paint. Maintain the topcoat, don't let the epoxy primer be exposed to UV.