skim off those that float, as these are useless for bait
Not true. There are many ways of using floationg caster, and some of the best matchmen I know keep hold of the floaters. It might be a tiny difference, but a caster with a steel hook in it sinks at a slightly faster rate than one without.
This effect is even more pronounced when using fluorocarbon, which adds to the sink speed. Using a floating caster will counter some of this effect; as long as it sinks on the hook, it doesn't matter. 'On the drop' bites are easier to get on the floater, too.
If you want some of the best pastie fishing you've ever had, collect a few pints of casters from sunday matchmen after the match (you will need a few quid's worth) and then let them turn in the fridge the rest of the week.
Most will be floaters, some will be sinkers. Make up a waggler rig with all the weight right under the float and a 2ft tail of 4lb fluoro to a forged size 18 or 16. Or get a float controller. You're going to fish your baits at distance, just under the surface.
Get to the lake early, and bomb it with the casters in a brown crumb binder, or better still with sticky mag. Wait until a few carp come to scoff the caster, then get some more in. Wait until the carp come back, and bung some more in. Keep that up for a couple of hours or more, until the water's alive with the little beggars.
Two catsers on the hook, cast into the middle of this maelstrom, and prepare for fireworks. I've seen people have 200lb off the top this way.