This part of ISO 16938 specifies a method for determining the staining of porous substrates (e.g. marble, limestone, sandstone or granite) by sealants used in building construction. The method evaluates the likelihood of a sealant causing an early stain on a porous substrate due to exudation of materials from the sealant. The outcome of the test is specific to the tested sealant and tested substrate and cannot be extrapolated to other sealant formulations or other porous substrates. During this accelerated test, if the sealant does not stain or discolour the substrates, it does not mean that the tested sealant will not stain or discolour the tested porous substrate over a longer time period. Experience in various countries with similar test methods has shown that the compression of test specimens further accelerates the occurrence of staining.