Addition of nanoclay (0.5 and 1 wt % ) increased noticeably the viscosity to the UPR resin. The gel time of the UPR resin was significantly decreased by adding small amounts of nanoclay due to the interactions between the filler and the styrene in the UPR resin. Moreover, addition of nanoclay decreased the shrinkage degree during curing of UPR resin. In the cured composites, improved thermal properties in UPR were reached by adding nanoclay due to the creation of a network between the filler and the polymer matrix. Bundle nanometric size nanoclay agglomerates were observed which did not affect the glass transition temperature but increased the viscoelasticity of the UPR–nanoclay composites. Furthermore, the improved properties of UPR by adding nanoclay produced enhanced impact strength to coated natural stone pieces, as both stiffness and toughness were improved by nanoclay addition. Finally, good adhesion of nanoclay filled UPR to the surface of natural stone was reached.