NOVELTY - Sol-gel paint comprises composition of: (i) black spinel type pigment providing absorption of solar radiation; (ii) open polyhedral oligomeric silsesquioxanes; (iii) solvent, preferably pigment dispersions from modified pigment particles in alcohol compounds, ethers, esters or their mixture; (iv) polyfunctional silane acting as a binder which provides mechanical and thermal stability; (v) other organic pigments enabling the modification of color of selective coating; and (vi) catalysts for starting hydrolysis followed by condensation of polyfunctional silane.    USE - The sol-gel paint is useful for providing a thickness sensitive spectrally selective coating on metallic substrates applied by wet deposition techniques for facilitating the harvest of solar energy in the form of heat in solar thermal systems where metallic substrate is preferentially selected from, aluminum or stainless steel, a metal alloy applied by spraying, dip-coating or coil-coating and slot casting, preferentially by coil coating technique in that dry layer of thickness between 500 and 1500 nanometers is created (claimed).    ADVANTAGE - The thickness sensitive spectrally selective coating exhibits any combination of one or more of the following properties: (i) does not peel from the metal surface or crack immediately after curing even when a thickness of a paint layer exceeds 4 microns; (ii) consistently stays black even as a thin coat of less than 300 nm when painted onto aluminum coils; (iii) thermal durability at high temperatures such that it substantially does not ablate over a time period of 20 days when maintained at 600 degrees C; (iv) mechanical durable with hardness at least HD3; and/or (v) a thin coat of less than 1000 nm protects aluminum from ambient moisture (claimed). Surfaces exhibiting high solar absorptance, low thermal emittance, and high spectral selectivity are provided.    DETAILED DESCRIPTION - Sol-gel paint comprises composition of: (i) black spinel type pigment providing absorption of solar radiation; (ii) open polyhedral oligomeric silsesquioxanes, which provides the modified black spinel pigment particles in dry form; (iii) solvent, preferably pigment dispersions from modified pigment particles in alcohol compounds, ethers, esters or their mixture; (iv) polyfunctional silane acting as a binder which provides mechanical and thermal stability; (v) other organic pigments enabling the modification of color of selective coating; and (vi) catalysts for starting hydrolysis followed by condensation of polyfunctional silane. An INDEPENDENT CLAIM is included for thickness sensitive spectrally selective coating where the sol-gel paint is applied, where spectral selectivity is not greater than 0.90 solar absorptance (as) and thermal emittance (et) is not less than 0.18; or thickness sensitive spectrally selective coating where the sol-gel paint is applied, where spectral selectivity is 0.90 as and et is not lower than 0.18 with color strength from Casterisk=2 to Casterisk=10.