Do you know of any platinum solder that does not drag out of
solder seams and matches the color of platinum well?
Precious Metals West came up with a series of plumb platinum solders
after noticing that the late Stephen Kretchmer's heat treatable
(hardenable) platinum alloys melted at temps a couple hundred degrees
under standard platinum alloys.
They make a Hard, Medium, and Soft grade of solder based on those
alloys as the starting point, with platinum percentages at least 900
and going up for harder grades. These solders are an almost perfect
color match for standard platinum alloys, (the soft is very slightly
darker, and the hard very slightly lighter in color than 90/10
iridium platinum) and are the same hardness, so the seams neither
show, nor pull out in polishing. They are slightly more difficult to
use, however, since they don't flow as easily as standard solders.
They are a bit more sluggish, and often, even though fully flowed,
leave what appears to be a solder scar that looks like it should be
remaining unflowed solder. So in use, place the solder paillons where
you will be able to clean up the solder scar.
After that, these are great solders, especially for things like ring
sizings, or less complex joints or any other times where you want
maximum strength and minimum visibility of the solder. While PMR
makes these, I'm pretty sure various other tools/supplies/metals
dealers now carry those solders too.