Does not need a primer for normal conditions. Two-pack polyurethane paints are used almost exclusively as topcoats as they offer excellent UV resistance as well as providing a high gloss and enhanced durability. These characteristic come from the fact that as well as physically drying like traditional single pack paints; the two components (a base and a curing agent) undergo an irreversible chemical reaction. This chemical reaction relies on the two components being combined together in the right quantities so it is extremely important to adhere to the correct mixing ratios, (4:1:1) 4 Parts Paint, 1 Part Hardener, 1 Part Thinner.
Polyurethane coatings can be applied to many surface materials, including wood, metal and plastic, and virtually any type of surface. Its versatility of use is one of the characteristics of polyurethane that has led to its popularity.
A primary benefit of using polyurethane coatings is the protection it provides the surface itself. When applied, the coating creates what is essentially an impermeable barrier between the elements and the object it's covering. With its smooth, hard surface, polyurethane seals out rain, dirt, mud, and other natural surface enemies. It allows provides protection from humidity and oxidation, which can cause corrosion.
Since polyurethane doesn't emit any vapors that may be harmful to people, animals or the environment, the choice to use it comes at little risk overall. Just be cautious that when you do use it you don't allow children or animals near any open containers since the consumption of polyurethane, like any other coating material, is harmful when swallowed.
Polyurethane has come a long way since World War II, and with the many benefits and advantages it provides, it's no wonder it has become a mainstream choice in surface protection.