Cleanliness is key, and cross contamination of particles and materials can further ruin the paint. Minute fragments can actually be recycled in the water you use and reapplied and smashed into your paint. This can lead to a larger job than originally anticipated. To combat this, soaking the wet/dry sandpaper in water will do the trick. The water will soften up the paper. Plus, contaminants will be washed off and the paper can remain clean.