Use ‘dry’ for rough sanding or for things that you don’t want to swell (e.g. wood, etc.). Otherwise try to use ‘wet’ (with water) – as this reduced friction and also creates a ‘slurry’ from the particles you have sanded off, which helps polish-up the thing you are sanding. Plastics might need up to 1000/1200 grit to get a smooth enough finish to polish. (Available on ebay)
