In order to provide a second clue as to the inapplicability of LPS to UCS Stage II in the case of porosity not completely filled by water, now we discuss the existence of a grain boundary liquid film. To verify whether or not the said liquid film exists, we will refer to a previous work by Kwon and Messing [39]. They analysed the liquid film thickness in pressure-less liquid phase sintering. Using lubricant theory, we can calculate the squeezing speed of the liquid at the interparticle region in the early sintering stages (additional details are reported in ‘Supplementary Material’) [39]: