I was just reading a thread about builders cleans and how it had to be done twice as it was mega filthy. Have any of you ever heard of the TBM or two bucket method that car detailers use.
Simple really, they use one bucket containing their wash solution and one with clear water and a grill in the bottom of the bucket about 6" off the bottom. They fill their sponge in the cleaning solution, then onto the car, when they need more solution, they first rinse the sponge in the clear water bucket against the grill.
All the crap from the car ends up in the bottom of the rinsing bucket, and the cleaning solution bucket stays clean. Instead of dipping the sponge in the cleaning solution and possibly transferring the muck back onto the car. Next time your cleaning your car/van try it, you'll see how much muck is in the rinse bucket.
I would think the theory should work well with a builder clean.
If you want to read more about it have a look at detailingworld .com