Question About Soldering

[whitmore]

I'm fabricating the fuel filler & foot valve assembly... three pieces of 1/8" brass pipe. Two of the pieces are cut with a saddle joint, to be soft-soldered to the third piece. My soldering skills are OK, but not great. I would like this to look nice when the job is finished. So, my question is, is there something that can be applied to the pipes to prevent solder from sticking

Well, yes, there's anti-flux.
<https://www.amazon.com/Tix-Anti-Flux-1-2-oz/dp/B013PYRJK6?>

What works for me, though, is just applying a little flux (stainless steel flux is good for brass, rosin is NOT),
and tinning the surfaces to be joined. A Q-tip dipped in acid flux to wipe the surfaces, holding a
solder-charged iron against the surfaces for a few seconds, and the tinning is done. Then, wash off all the flux.
Position the loose parts (in your case, so they fit by gravity)
and apply a torch flame (not too close) until the solder reflows. If you want to be fancy, you could
consider making preforms of copper foil (available at any crafts supplier for stained glass) that put a kind of gusset plate
over the joints; some kind of T-section form with copper laid over it, and a neoprene pad over, and a hydraulic press,
can cold-form a solderable additive element (like the lugs on a brazed bicycle frame).

Hard solder (silver solder) is not easy with brass, because the melting points are too similar. I'd try to avoid that.

For tidying-up afterward, a buffing wheel might be useful. Large solder blobs go away with a few swipes of a file...

 

[mikey]

Soot deposits from an acetylene-rich torch flame will reduce adherence of silver solder. Apply the soot, wipe the soot away where you want the solder to stick with some alcohol and solder.

 

[David S]

John do a search for liquid solder mask. There all sorts of options, typically used in the printed circuit industry.

David

 

[JPar]

Thanks to everyone who replied.

I tried an experiment using both graphite (soft lead pencils) and liquid paper (typewriter correction fluid) on an extra piece of brass pipe. Both seemed to inhibit the solder flow, but the graphite is tricky because it comes off easily as the part is handled. My Dad used to say, "God hates a coward", so I decided to proceed using the liquid paper. It took a couple of tries, but I got the job done and it doesn't look too bad.

The first picture shows the piece immediately after soldering, still in the "quick & dirty" fixture I made to hold the parts. It looks messy because it still has the liquid paper and some excess solder.


The second picture shows the assembly after some cleanup, attached to the gas tank and filled with water for a leak test. It passed! It just needs a little more cleanup now.

 

[brino]

thanks for showing your results.
I will add that trick to my list.
-brino