Hi Ash,
I've just done a set on the front wheel of the bike I got back from Hull. They were in fairly good nick to start with but I cleaned them up on the lathe with emery tape, one end at a time. Take care not to wrap the tape around the spoke but just rest it on the surface. Then with a wire brush in the pillar drill I went over each one until satisfied. The nipples were put on a spoke and cleaned in the pillar drill, one at a time. Next was a dip in brick/path cleaner (dilute hydrochloric acid) for about ten minutes to remove any bits of rust and original zinc. Rinse off and then one minute in caustic soda, rinse again and you're good to plate.
I like to be in control of as much as I possibly can - obviously I have to farm out chrome plating but I have used Prestige in Mexborough for years now with excellent (but slow) results. They did a brilliant job on my last set of DID rims with all stampings still legible.
There are some pics on my site in the What's new section
Ian