I was reading about "most", "most of ", and all examples that I read they were in plural. For example: "Most companies offer...", Most of my classmates study..." and I note that all those examples are in plural. But I have an exercise that say : "Today, most oil paint (is/are) produced in factories. I don´t know which one I have to use, because I did not find an example with a singular noun, but I know that the verb and the subject has to be in the same number.
What you're referring to are uncountable (or mass) nouns. These are nouns which have only one form, and are not changed to become singular or plural. They often appear singular, but have a meaning that is really neither singular or plural. Mass nouns are often liquids (water, coffee, juice), gases (air, steam, mist), powders (sand, flour, dust), or abstract concepts (data, information, politics). Paint, used in your example, is a liquid, so it is an uncountable noun.
Uncountable nouns seem to me to conceptually be closer to a plural noun than a singular noun. We can compare quantities, for example. We can say "there is more water in my bottle than in yours." As in your example, they can be used with "most," like a plural noun can.
But grammatically, they are typically treated as singular. They take singular verbs, and would typically be referred to by singular pronouns.
Some examples of similar sentences to your example: "Most water on Earth is salt water." "Most of the air we breathe is nitrogen." "Most dust in peoples' homes consists of dead skin." "Most of the information I found online was useful."