I’ve been doing overlay work since the early 70’s, and it’s not that
hard to do. I’m no fan of pre-soldering the top or bottom sheet being
soldered together, and haven’t liked working with paste solders. Way
too messy. Usually, the bottom sheet of a traditional overlay has
some kind of texture applied to it, and with intricate piercing,
nearly impossible to texture evenly that recessed background after
soldering. So texture the background with a diamond bur, a Florentine
liner, or something before soldering, would be my suggestion.
My biggest concern is that the two sheets to be soldered together,
the pierced top sheet and the bottom plate, are absolutely flat
together before soldering, or even fluxing. I flux the two sheets
while they are flat together. I always make sure there is an excess
of back sheet around the outside of the overlay being soldered, as a
"shelf" for the solder pieces I place there. After soldering, this
lip is sawn off and filed smooth.
After I flux, I will carefully warm the overlay to be soldered to
dry out the moisture in the flux. If you don’t do this pre-warming
step, your solder pieces will fly all over the place as steam
develops during heating.
I always solder with a bushy flame, working underneath the overlay.
Use a heavy stainless steel screen on a tripod, as an example. I like
a Prestolite acetylene torch for this purpose, not a torch with a hot
If you heat in an even, circular manner, and your solder chips are
placed around the outside of your overlay, touching the edge of the
top sheet, you should have terrific flow into the interior of your
pierced top sheet, with no solder puddles on the visible back sheet.
You may need to do several solderings to get everything completely
soldered. Use a medium solder, if you are soldering silver, as hard
solder just requires too much heat to flow in the manner needed. Oh,
and if you are soldering silver, a fabulous heat conducting metal,
turn up the heat! You’ll need plenty.
I also am not a fan of using clips or cross-locking tweezers to hold
overlay-type soldering together. In my experience, the clips are
huge heat sinks, and because the metal can soften when hot, they are
apt to leave marks in the surface of the metal being soldered.
Hope this helps!