A model is presented here to predict fluid reflow in a viscous liquid film having time-dependent physical properties. The model is applied to the practical example of the reflow planarization (surface levelling) of 'edge bead' features resulting from the spin coating process. During the reflow planarization of edge beads, the physical properties (surface tension, density and viscosity) of the spin-coated liquid can change over time for several reasons e.g. temperature change, solvent loss, and chemical reaction. The model is compared to experimental findings obtained using two spincoated viscous liquid films: a drying photoresist (SU-8) and a curing elastomer (polydimethylsiloxane/PDMS). The former reveals the competition between reflow planarization and solvent loss-the latter reveals competition between reflow planarization and polymerization. It is demonstrated that the changing physical properties of the spin-coated viscous liquid, especially the viscosity which can change by orders of magnitudes during these processes, impose limitations on the ultimate achievable practical planarization of the film.