I seem to remember that new Indian tanks had a light coating inside, of what turned out to be primer and general overspray..
Mine had a brownish red coating thickest under the filler hole and thinning out further back..
Its still there now after fifteen years so i wouldn't be getting too aggressive with it thinking its rust..
Unless your going to use a sealer i think a wash out with paraffin and oil mix, like they use on steam engine motions would suffice..
Would certainly be easier to dispose of then dirty petrol..
Incidentally I use brick acid to clean heat exchanger waterways, and was refused sale at my local builders merchants the other day whilst there safety procedures where under revue..
Seems the feckwits of London have been throwing it in peoples faces and parliament are going to knee-jerk in their usual way and ban/restrict sales, or try too..
May be an idea to get some in stock before it goes the way of the old Nitromores paint stripper, completely useless stuff...
https://www.theguardian.com/uk-news/201 ... f-products