The efficiency of solar thermal concentration systems depends on the properties of the solar absorber. Solar receivers need to accomplish high light absorption and good durability at elevated temperatures. In this work, the commercial Solkote paint has been optimized for its use as Thickness Sensitive Spectrally Selective (TSSS) absorber paint by dip-coating method for medium temperature solar thermal applications. Both the extraction speed and the number of layers to which the Solkote paint could be applied by dip-coating to register promising optical selectivity properties (high solar absorptance value, αs; and low thermal emittance value, ε_T) have been established. All the experiments have been carried out using the commercial Solkote paint as received without any pre-treatment (dilution or concentration). Additional experiments based on applying a SiO₂ layer onto Solkote paint layers have been carried out in order to study the protective action that SiO₂ could have on paint coatings.

All coated samples have been analysed through tests of thermal resistance (400ºC, 200h-800h) in a conventional circulating air furnace and constant condensation (40ºC, 150h-600h) in a Q.U.V. weathering chamber according to the procedure developed by ISO 22975-3 (2014) standard. The performance criterion (PC) was calculated according to the ISO 22975-3 (2014) standard for defining acceptable durability or service life of the absorber.

All the samples analysed withstood both the thermal stability and the constant condensation tests regardless of whether or not they contained the silica layer on top. The greatest variations in the curve shape were observed after the first accelerated ageing stage, i.e. after 200 h at 400 °C in the case of the stability test and 300 h at 40 °C in the case of the condensation test. In all cases the optical performance of the absorber surface was reduced less than 5% of its original value during the design service life time period. In fact, in all cases the PC value registered was zero. Therefore, the both kinds of samples analysed are qualified as far as thermal stability and condensation conditions are concerned, regardless of having an upper SiO₂ layer. This statement demonstrated that the both paint layers are enough to resist weathering and there is no need to use an upper protective silica layer.