I've been a bottle collector and digger for a while now, something like 15 years and ive refined a good way that works for me, i used prass lock pins that I got from my friend who is a locksmith and use them to clean the inside with water and swirl them about. I have found that using gravel and aggregates scratch the glass but brass being soft and the pins being rounded at the ends doesnt do this. I also use those green plastic type scour pads, im not sure what they are called, but these remove the rust stains. Strong acids can be quite dangerous and I have never ventured into this. there also is tumbling as previously mentioned, would be more accessable in the US than over here, I have afew of my better bottles tumbled and they looks sparkling, only danger is some bottles can be over-tumbled and this makes the embossing rather faint. Some people have spray laquered their sick bottles and this can get nasty and the laquer can go an ugly shade of yellow, I bought a bottle like this recently and spent ages cleaning the crap off it. Hope this helps you