So, do some experimentation, and find out what kind of stop-off works best for you.
But —- VERY IMPORTANT — don’t let the easy availability of stop-offs lull you into carelessness in the brazing process, as discussed below!
1. Over-use. Because of the effectiveness of many stop-offs, a number of brazing shops tend to discount the need to control the amount of BFM they apply to parts, thinking: “Oh, don’t worry about how much BFM is used, just put on more stop-off”. That is a VERY bad idea! Applying only the correct amount of BFM is ALWAYS important!
STOP-OFF SHOULD NEVER BE USED AS AN EXCUSE FOR USING TOO MUCH BFM!
Unfortunately, I see too many shops using too much stop-off, rather than properly training their people to back-off from such overuse, and training their personnel, instead, to use only the correct amount of BFM that is needed to make a good brazement. When the proper amount of BFM is used (the volume of BFM used — when it completely melts and turns to a liquid — should be approximately 150% of the volume of the joint to be filled), this amount of applied BFM may then justify using only a very small amount of stop-off, or it may permit them to get away from using stop-off altogether.
2. Correcting stop-off “mistakes”. It is not uncommon for persons applying the stop-off to make a mistake, and accidentally apply stop-off onto a surface that needs to be brazed. I’ve heard some folks very softly say: “Uh, oh…” as they’re putting stop-off onto parts, and then, when I look in their direction to see what they just did, I see them taking a cloth and dipping it into a solution such as acetone or alcohol, with the intent of wiping off the stop-off from the surfaces where the stop-off is not supposed to be. THIS DOES NOT WORK! Please understand that the following saying is very true: “Once stopped-off, always stopped-off”. You cannot merely wipe the stop-off from the surface and then expect that the surface will then become brazeable once again. That is incorrect thinking! Residues from that stop-off are still there in the nooks and tiny crevices of the “surface roughness” of that component, and these tiny residues of stop-off can actually prevent the BFM from properly wetting that surface.
The only way to effectively remove stop-off from surfaces is to either thoroughly ultrasonically clean that surface, or machine off that top surface layer that was contaminated.
BETTER YET! Find ways to eliminate the use of stop-off altogether. Too many shops plan on the use of stop-offs on too many of their parts, believing that it is always wise to use stop-off. No, it is not.
Brazing stop-offs can be a helpful tool for a brazing shop, but it should be used sparingly, and only when absolutely needed. Otherwise, overuse of stop-offs can actually result in scrapping parts unnecessarily or create a lot of extra work to clean off surfaces for subsequent brazing. Stop-offs can be an effective tool for brazing when used sparingly and wisely.