Review on Harris S0FB01Type Solar Welding Flux by Otto Suarez

Works the same as gas backflow. Dissolve it in methanol and apply paint. In a few minutes the methanol will evaporate and you can start welding. Slag is hard to remove unless you have access to a rear hatch or use a terrible HF and nitric acid passivator to remove it. But it works great, prevents shugaring, easy to use.