The accuracy of sink-float separations in water-based media such as Magnetic Density Separation is compromised by tiny air-bubbles attached to the surface of hydrophobic materials in the feed. Separations of plastics mixtures with accuracy better than 10 kg/m 3 require a total elimination of air-bubbles by pre-wetting the surface of the feed particles with a thin film of water. Theoretical and experimental results are compared for pre-wetting processes based on condensation from the gas phase and direct liquid-solid contact for both shredded waste and virgin polymers. The tests show that the wettability of most polymers improve slightly by steaming for about 1 minute, but the best wetting results are obtained with immersion in boiling tap water. The success of wetting in boiling water can be explained by the deposition of a thin layer of calcium carbonate. Shredded plastics are easier to be wetted than virgin polymers, probably because a relatively rougher surface is caused by the shredder process. A new wetting process to eliminate air bubbles was developed on the basis of these results, which carries a minimal amount of heat and water with the feed to the MDS process liquid.