Although wetting transitions have been observed for helium and hydrogen on alkali metal surfaces, no finite temperature, quantum simulation studies of these systems have been able to conclusively locate a wetting transition. This paper presents such calculations for the systems ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ on Rb and Cs, using semiempirical hydrogen-hydrogen interactions and ab initio gas-surface interactions. Comparison with experiment implies that the adsorption potential is $\ensuremath{\sim}10%$ more attractive than is predicted by current theory. Simulations of ${\mathrm{H}}_{2}$ adsorbing on a 15 \AA{} thick film of Rb on Au predict that this system will show a decrease in the wetting temperature of about 1 K compared with ${\mathrm{H}}_{2}$ on pure Rb. The simulations reveal that a commonly used ``simple model'' is surprisingly accurate.