This study concerns blinding of the filter cake during filtration of wastewater solids. A theoretical model is suggested that describes blinding as happening in the filter cake only and not in the filter medium. It is based on the idea of having a filter cake built up by larger particles. This filter cake then serves as a secondary filter for small-scale solids. Data obtained from filtering wastewater solids show good agreement between the estimated parameters for the amount of small-scale solids and the degree of blinding. This indicates that the model is plausible.