TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The mixing time for the preparation of the sol mixture is 1 minute to 7 days. The impact time of the sol mixture on the substrate is 0.1 second to 5 hours. The separation of the coated substrate from the sol mixture takes place with a speed of 0.1-100 cm/second. The drying time of the coated substrate is 1 second to 36 hours. The heat treatment takes place at 70-300 degrees C, preferably 85-250 degrees C, and for 1 second to 5 hours, preferably 1 second to 4 hours. The application of the sol mixture on the substrate takes place by: immersing the substrate into the sol mixture, spraying on the substrate, roller coating of the sol mixture or applying through a slot nozzle. The coating takes place discontinuously or continuously, preferably coil to coil. During coating, the value of the thermal emission coefficient of the coated substrate is adjusted to a value, preferably less than or equal to 0.35 against the thermal emission coefficient of the substrate. Preferred Components: The mercaptosilane compound is of formula (HS-(CH2)n-Si(R1)m-(OR2)r) (I). The end concentration of the mercaptosilane compound in the sol mixture is adjusted to 0.01-50 mass%. The sol mixture further comprises at least a hydrolyzable silane compound. The end concentration of the further hydrolyzable silane compound in the sol mixture is adjusted to 0-70 mass%. The further hydrolyzable silane compound is of formula ((R3)n-Si(R1a)m-(OR2a)r1) (II). In the process, a further hydrolyzable, preferably doubly endcapped, silane compound of formula ((OR2a)r2-Si(R1a)m1-R4-Si(R1)m1-(OR2)r2) is used. The solvent is aliphatic alcohol, preferably ethanol, 3-butoxy-2-propanol or propyleneglycolmonobutylether. The acid catalyst is hydrochloric acid, hydrobromic acid, hydriodic acid, nitric acid, sulfuric acid, orthophosphoric acid, formic acid, acetic acid, propionic acid, butyric acid, salicylic acid, trifluoroacetic acid, trichloroacetic acid, trifluoromethanesulfonic acid or methanesulfonic acid. The amount of: the acid catalyst is 1 mu mol to 1 mmol and water is 0.001-80 mass%. The substrate comprises a spectral selective solar cement, preferably a chromium oxynitride, titanium oxynitride or titanium-aluminum-oxynitride-absorber coating, which is separated from the polished metal surface comprising copper, aluminum, iron or its alloys. The thickness of the protection film is adjusted to less than 1000 nm, preferably 5-500 nm.R1, R2 = 1-14C-alkyl or aryl;n = 0-20; eitherm, m1 = 0-2; andr, r2 = 1-3; andr1 = 1-4; andn = 0-3; orm+r = 3; orm+n+r = 4; orm1+r2 = 4;R1a = 1-10C-alkyl, 1-10C-cyclaolkyl or aryl;R3 = alkyl, cycloalkyl or aryl (all optionally substituted and fluorinated); andR4 = terminated or substituted alkyl chain, polydialkylsiloxane, polyether chain, or perfluoroalkylated chain. EXAMPLE - 3-Mercaptopropyltrimethoxysilane (4 g) was dissolved in ethanol (25 g). The solution was acidified with 0.1 M hydrochloric acid (0.1 ml) and stirred for 4 hours, during which only hydrolyzed monomer and dimer were formed. During the hydrolysis, the sol was diluted in addition with ethanol (75 ml) and applied on a commercially available sunselect material. After an influential time of 240 seconds, the sample was immersed on the sol and the monomer and dimer were penetrated into the porous structure of the material. The sample was extracted form the sol at a speed of 10 cm/second, air-dried for 60 minutes and heat treated at 140 degrees C for 60 minutes.