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One of the surface treatments of paper is surface sizing. In this operation, hydrophobic polymers are added to paper in a size press, usually dissolved in an aqueous starch solution. We investigated the penetration of starch solutions in paper in a puddle size press. Starch penetration in paper was followed by x-ray microscopy. A mathematical model was developed, which explains the observed penetration depths. The model is an extension of the classical Lucas–Washburn theory, modified by a build-up of pressure inside the sheet during liquid penetration. The model predicts liquid penetration as a function of paper speed, nip load, viscosity of sizing solution and porosity, and surface chemistry of paper. [ABSTRACT FROM AUTHOR]