Does footvax give them protection against scald? I thought scald was a different organism; it often leads to footrot, I know, so footvax is quite possibly a good precaution, but I'm not sure it'll help clear up the underlying scald.
You'll improve the scald if you can put lime down where they congregate - around troughs for instance, gateways, etc. And turn off the rain and get the ground to dry up...
We've discussed using formalin (trot them through a 2% solution - that won't harden their feet too much; but you may need to repeat every 2-3 weeks while it remains wet underfoot) and zinc sulphate (they must stand in it for some minutes) before; the washing up liquid is a new one on me, I'll try that sometime! And yes, the cleaner their feet are before entering the bath, the more effective the bath will be, so a 'prewash' is a great idea if you can do it.. If you can have them stand around on cement for 30-60 mins afterwards it'll be more effective.