Here is a tip for preventing solder flow from @hannah.lynn.jewelry - “Ochre powder is a paint pigment powder that can be found in most art stores for a fraction of the cost of some of the solder flow preventing jewelry products that are available.
While marker and pencil can work well in some uses, I find this to be helpful with highly detailed areas that you may not be able to draw on. I use it when making a textured piece or on items with granulation in which the granules are fused, not soldered. Solder would flood the granules and ruin the effect. However when it comes time to solder the granulated component to a backing the Ochre comes in handy. Flux your piece and let it completely dry. In a small dish or shot glass add a bit of Ochre powder and drops of water little by little until you get a thick mustard like consistency. Apply it to detailed areas with a fine paint brush where you do not want solder to flow or flood into. Allow it to completely dry before soldering. When heated the Ochre will be thick and turn black, preventing solder from flowing. Once you're finished pickle as usual then scrub the remaining ochre off with soap and a brass brush. Works like a charm! Rio has a very similar product called "No Flow" which also works great but Ochre powder is less expensive.” Hannah thank you SO much. Folks Hannah sent this back in 2019, sometimes emails get missed in the shuffle. If you’ve emailed a tip feel free to follow up :)