Thoughts on flux application. Almost all beginners make the same predictable mistakes in how there approach putting on and drying flux as well as how they try to braze. I’ll just touch on a couple of things. A full description is way beyond one post and why I teach frame building classes. There are a lot of interconnecting dots with application and drying and preheating and the pattern needed for brazing efficiently. If something starts to go wrong in one area it can lead to failure in others.
Flux is the rookie’s best friend. It keeps the metal clean and helps the silver to flow. And it lets one know when things are at the right temperature to start brazing. Once it gets used up during the braze, there is real trouble. For starters it needs to be understood that a beginner will take longer to braze a joint than a pro – sometimes a lot longer. For example it probably takes me 3 to 5 minutes to braze a lug while a typical student takes maybe 15 to 20 minutes (with me constantly suggesting refinements). That means they need more flux so they don’t run out.
Here are 3 suggestions. 1st, don’t pile it all up just around the lug because it won’t stay there. Spread it out up the tubes. Richard Sachs’ amount is perfect for him but a rookie should put even more up the tubes where it can be accessed if necessary. 2nd, if you are using a small flux brush, roll the brush so the flux stays on in greater volume instead of brushing it on which only leaves a thin layer. And 3rd, if your drying flux technique is poor and it ends up on the floor instead of staying near the joint, stop and let it cool and add some more before continuing.
I might add that a beginner’s instinct on how to dry flux is usually flawed. One needs to stop and think how can I use my flame to keep as much of it in place as possible? It is enviable that some will not stay where it is useful but if the majority is not where it needs to be then the builder needs to think how they can improve their poor flame pattern.
The 2 kinds of flux I use in my class is Stainless Light by Cycle Design and Type G by Gasflux. I prefer these 2 because they have a higher temperature range over typical white flux so they forgive rookie mistakes better. They are also thicker than the watery stuff I buy at my local welding supply store. The methods used to dry each type of flux is very different from each other.