I think you're always going to get some surface rust on your chain in the winter, but it's not a problem if you keep on top of it. I lubricate mine weekly - I clean monthly. I got a Motul kit for £15, it had a decent brush, chain cleaner and chain lube in. Got all the rust off mine perfectly. As far as ACF50 goes, don't put it on your brakes at all and you don't need it on your chain/sprockets either. From there anything metal is pretty much OK to give a coating, Obviously aluminium parts won't rust anyway but I give it a little coating. Gear linkage usually gets some rust on so I pay particular attention to that area, I've also got flakey paint on my exhaust so I give that a good coating too. You can brush it on if you want, I just spray it all over though.