Important factors influence the performance of surfacing seal, or surface treatments, such as the loss of voids that results from the wear of the aggregate and the effect of traffic on the embedment of the aggregate in the underlying layer on which it is constructed. Research results are given that indicate the likely reasons why single seals are generally more susceptible to fatting-up than double seals. It is shown how the modified tray test can be used to avoid this problem. A rational approach to the design of single and double seals that incorporates the actual measured void content instead of an accepted average quantity is outlined. The accurate determination of the stone spread rate with the modified tray test is also discussed.