For paint/coating materials, we cannot directly test the wet samples. The paint/coating needs to be applied onto a substrate material for testing.
For thin paint/coating layers, they are translucent to solar radiation and shown below is an example. Some solar radiation can penetrate through the thin paint/coating, reach the substrate material surface, and be reflected or absorbed by the substrate.
The substrate material has some influence on the SRI test results, though the influence is a secondary factor and weak in general.
For paint/coating on building exterior wall or roof, we recommend using cement board as the substrate, as this material is easy to obtain and is close to the plaster layer on the wall or roof.
On the other hand, due to the weak influence, it is okay to use other plate materials as the substrate (e.g. aluminium plate, gypsum board, glass plate). For typical paint/coating layers with a few hundred micrometers of thickness, the influence is close to negligible.
There is no specific requirement on the paint/coating thickness. Technically, we can test any flat surfaces, regardless of the paint/coating thickness. The manufacturer’s instructions should be followed so that the sample tested is close to the actual installed materials.