Test with methanol
Attach / tape a soaked cloth a few minutes to the surface. If dissolved / the paint becomes discolored, it is an acrylate or latex paint (acrylate is a latex paint). Methanol does not disolve alkyd or linseed oil.
Test with lye (NaOH in water)
Put on a cloth soaked with lye and wait a few minutes. The lye dissolves (saponifying) alkyd - and linseed oil paints. Old linseed and alkyd oil color becomes brittle.
Make the water test. Splash water on the surface. If the surface is becoming wet and water tend to be sucked into the surface it is suitable for silicate painting. But if water drops are formed and rejected there are too many water repellent substances such as plastic, oil or grease in the surface.
Water glass reacts chemically with the carbon dioxide of the air and the mineral substrate to a microcrystalline structure that is completely open to diffusion of moisture and carbon dioxide. Acrylate adhesive binds to the surface as a glue and forms continuous plastic film that prevents moisture and carbon dioxide from diffusion. Acrylate is degraded by the sunlight, resulting in a shortened life span of the paint.