Wouldn't go as far as saying it's a top tip, but worked in this case... Porsche 924 expansion bottle / bracket that came attached to an airbox, it was the Kjet gubbins I was after so everything else was just secondary. Still it was far to good to bin, even it's of no direct use to me, so gave it a clean up. Had the usual rusty stains / tide marks and discolouring you'd find inside any tank that's this old, previously I'd have spent an age poking a stick / sponge or similar through the neck to clean it, but this time tried shaking a chain around inside instead (in a bleach solution) and the below is the result, came up well I thought. (Chain's easy to retrieve too) May or may not work for you, but a lot less effort if it does and can't hurt to try.
