When the rails were installed on the slip, I then covered the slip with two layers of 2mm plastic sheet between the rails (got loads of it). The rack system was cut from "Evergreen models" plastic lapboard sheet and located between the two centre rails. After masking off the rails and the rack system, I then sprayed the slipway with textured concrete coloured paint. This can be purchased from B&Q. Now studying lots of slipway pictures for the paint splashes and general grime that the slipways accumulated with their workload.
The idea came about after seeing AlanD's dock wi slipway. I then thought, surely there's no better way to display a model out of water.