Abstract: Provided are methods of forming stacks comprising a substrate and one or more sol-gel layers disposed on the substrate. Also provided are stacks formed by these methods. The sol-gel layers in these stacks, especially outer layers, may have a porosity of less than 1% or even less than 0.5%. In some embodiments, these layers may have a surface roughness (Ra) of less than 1 nanometers. The sol-gel layers may be formed using radiative curing and/or thermal curing at temperatures of between 400° C. and 700° C. or higher. These temperatures allow application of sol-gel layers on new types of substrates. A sol-gel solution, used to form these layers, may have colloidal nanoparticles with a size of less than 20 Angstroms on average. This small size and narrow size distribution is believed to control the porosity of the resulting sol-gel layers.