Why is there rust in oil tank? and how do I clean it?

[DerekAJS30]

My recently purchased Model 30 seems to have encountered significant engine issues in its lifetime. It’s now a nut and bolt rebuild but I encountered something unexpected with the oil tank. First I noticed no oil filter in the tank (doh !), but then it occurred to me that the oil filter is there for a reason so I washed out the tank with water and detergent to get rid any grot. Emptying into a white sink I noted quite a lot of “rust” and “grit”. I persisted with vigorous washes, but it kept coming. So, I decided to try Vinegar with some salt added to remove any rust. I filled the tank and left it for two days. Well, I emptied it and a mountain of grot came out. I’ve washed it again many times with detergent and water with vigorous agitation but still it comes. So now it's on its second vinegar soak.
BUT my point is: why would a tank that has spent its life almost full of oil have rust in it ? Is it water vapour in the oil from engine operation? and how do I guarantee the tank is clean and grot-free before I reconnect to my now hugely expensive rebuilt engine?
I won’t sleep until I have a proper solution so please help !!!