Details are given of extensive tests done to study the design and operation of desanding systems for oil-gas production separators. Sand and water are used in vessels of diameter between 0.9 m and 2.7 m. The use of non-dimensional groups, representing solids volume, wash time and wash jet momentum, permits generic scaling of the data. The fluidization factor, F, is defined which is found, for a given solid, to have a critical optimum value, higher values implying unnecessarily excessive washing and lower values less than optimum solids removal. The increase in critical F found between sand and Proppant appeared to be mainly due to increased particle size. Specific recommendations are made regarding wash jet design and deployment, the spacing and design of drain points and the use of discrete baffled wash zones. For large tanks multiple tiers of wash pipes are recommended. The optimum time to stop washing is identified and an overall design procedure is outlined. Although the procedure has been proven for washing relatively fine uncontaminated sand from production separators in the Forties field, further design information may be needed for washing alternative types of solids.