Most fluxes will bubble and turn light brown if they have reached the correct temperature. Drag the rod or wire of solder substance about the be part of, continuing to heat the area indirectly from the opposite side of your metal, or a nearby surface area. Torch soldering is ideal https://types-of-automotive-hose99382.activosblog.com/26846538/how-much-you-need-to-expect-you-ll-pay-for-a-good-types-of-solder-to-use-on-aluminum-radiator