Solder resist

Not really an electronics question but related. I'm trying to refine my technique for making pendulum suspension springs. Basically these are a strip of beryllium copper about 16mm wide which have thick brass "chops" soldered on each side of each end for mounting. The active length of the spring in between the chops is a few mm. The soldering problem is to prevent solder squeezing out from the chops when molten and forming littlee blobs on the active surface that could affect the uniformity of the bending. So I would like to put something on the spring surface between the chops that will inhibit solder wetting. This is the same sort of requirement as a "solder resist" on a PCB except that I will want to remove it after soldering.

Does anyone have any ideas for what I could use please? I've seen Tippex mentioned I think, I'm not even sure if that's still made! Soldering by the way is using electronic silver solder, either cored wire or paste.

tippex every time, make sure you use the red logo bottle not the green. when finished wash off with cellulose thinners or gun wash

Thanks Bernard - does it still wash off after heating or does it need scraping?

Jewelers rouge mixed in a little water and painted onto the surface you do not want the solder to be on has worked great for me - no issues with solvent or going off over time. Is removed easily afterward.

Washes off after soldering

Thanks for the input everyone. Actually I have a little pot of rouge somewhere, I'll give that a whirl first if I can find it. Tesco seem to have Tippex so I'll try that if the rouge doesn't work.

Why are you not riveting the chops to the spring?

regards Martin

Well, I have decades of experience with soldering and none of riveting! But Matthys recommends soldering to ensure that the edge of the chop is firmly in contact with the spring. There's some evidence that if there's a small gap then the CD of the pendulum can be unpredictable as the bend brings the spring into contact. I've also just recently replaced the spring on my new pendulum with one that has a wide single leaf rather than a double, and I took more care with soldering, and it looks like this on its own has significantly reduced the measurement noise I was seeing.

...and it has also increased the pendulum Q by 60%!

Sounds reasonable John. Thanks for the explanation.

regards Martin