TeEd, my faith in Halfrauds products is lacking after my silicone aerosol episode, but the mitt might be worth a try?
What I have done with my clay bar is:
1. I stretch it out so that it's about 4" x 2.5"
2. I palm it across an area such as the bonnet,turning it over until both sides are mucky.
3. I then take it in to the kitchen sink, put a spot of Fairy liquid on an old soft bristled pan cleaning brush, and gently scrub of the contaminates, whilst rinsing it under the tap with warm water.
Then when it's nicely rinsed I pat it dry quickly with a paper kitchen towel. I never fold the muck in.
This may sound like a lot of time consuming faffing around, but time is something I have in a plenty.
I keep it stored in a strong sealable food bag when I have finished.
Not sure what the Pre-prep spray is, but I guess it's mainly filtered water which acts as a lubricant?