PU (Polyurethane) thinner and NC (Nitrocellulose) thinner are both solvents used to thin and adjust the viscosity of paint, lacquer, and varnish. However, they have distinct differences based on the type of resin they are used with. PU thinner is used to thin and adjust the viscosity of polyurethane-based resins, while NC thinner is used to thin and adjust the viscosity of nitrocellulose-based resins, including lacquers.