The paper is concerned theoretically and experimentally with the flattening of asperities by tools in metal-forming processes, a topic important to lubrication in metal forming. It specifically studies the plane strain compression process. It proposes kinematically admissible velocity fields for the crushing of asperities on a plastically flowing foundation and by adjusting independent variables of the fields predicts the actual flows by an energy minimization method. The theoretical predictions are compared with plane strain compression tests on aluminium samples with surfaces abraded in the direction of bulk plastic flow (the condition that the modelling describes). Both theory and experiment predict that asperities are crushed at the start of bulk plastic compression, to generate a real area of contact a certain fraction of the tool area depending on the contact friction and aspect ratio of the flow field, between 0.75 and 0.95 according to theory and between 0.6 and 0.8 according to experiment; but that with further bulk compression these contact fractions do not change. Despite these quantitative differences, it is felt that the method developed in this paper could usefully be extended to study asperity behaviour in other forming processes such as drawing or rolling.