*2.4. Experimental Methods*

All samples were prepared by manually painting the geopolymer suspensions onto the underlying substrates using a brush. The microstructure of the coatings was observed on an Olympus SZ61 optical microscope and then on a 3D laser confocal microscope LEXT OLS5000 SAF (CLSM) and on a Vega 3 scanning electron microscope (SEM) from Tescan company (Tescan, Vega 3, Brno, Czech Republic). The thickness of the geopolymer liners was measured with a DeFelsko PosiTector 6000 portable coating thickness meter for metal substrates with an FNS type probe, with a measurement range of 0–1500 μm accuracy ± (1 μm + 1%) for a coating thickness of 0–50 μm, according to ISO 2360. The adhesion of the geopolymer to the chosen substrates was analyzed by the grid test method, according to ISO 2409 [36]—specifically, with the Elcometer 1542 grid test set.
