This part of ISO 5269 specifies two procedures for preparing a closed water system which is in retention equilibrium, to be used for the preparation of laboratory sheets from pulps with a high fines content. One procedure is applicable to the preparation of laboratory sheets using the conventional sheet former and the other using the Rapid-Köthen sheet former. The sheets prepared prior to attaining retention equilibrium are discarded. This part of ISO 5269 is especially applicable to mechanical and chemimechanical pulps, as well as to pulps prepared from recycled fibres, i.e. pulps with a high fines content for which retention of fines is poor when using the open water system as described in ISO 5269-1 or ISO 5269-2. This part of ISO 5269 is not applicable to some very long-fibred pulps, such as those from unshortened cotton, flax and similar materials. After retention equilibrium is attained, the preparation of laboratory sheets for testing the physical properties follows the procedures described in ISO 5269-1 (conventional sheet former) or ISO 5269-2 (Rapid-Köthen sheet former), whichever is relevant. This part of ISO 5269 is generally not applicable for the preparation of laboratory sheets from chemical pulps for which ISO 5269-1 or ISO 5269-2 are used. This part of ISO 5269 is not applicable for the preparation of laboratory sheets for measuring ISO brightness. These sheets can be prepared in accordance with ISO 3688[3]. NOTE The procedures for forming, pressing and drying are different when using the conventional sheet former and Rapid-Köthen sheet former, see Annex A.