I planted them in a 50-gallon trash can.

Then get a few gallons of Kilz and put down 2 coats.

Tank is usually blue and a little larger than a gallon jug. 

I'd go with a gallon of gloss Rustoleum paint and 4" brush total cost maybe $35

47 cents/gallon in a town in Michigan - http://fox17online.com/2016/01/17/no-joke-gas-is-47-cents-per-gallon-in-one-michigan-town/I paid $3 here. 

In a one gallon tank I usually use about a quarter cup of the grease remover.

She thinks the tanks are 10,000 gallon fuel tanks.

If you are painting the rest of the house yourself, get a 5 gallon bucket of paint-it will be less expensive and should be more than enough and leave extra for touch ups later on.   

I know there's special paint or coatings you can apply but that can be pricey per gallon.

I love how warm oil heat is , thats the good part , now , the furnace needs to be cleaned every year , tanks get gunked up , oil lines freeze from condensation , tenants run out of oil and system needs to be bled to restart , oil is $ 4.00 a gallon , so 100 gallon min delivery cost tenants $ 400.00 .