I am the manufacturer of the paste you're using. If you email me off
list, I would be happy to send you some of the original thinning
agent that we use to reconstitute the paste. If you prefer, you can
also use small amounts of mineral spirits to reconstitute the paste
in the short term, however, while mineral spirits will thin the paste
for use now, they will also dry out very quickly.
A quick note about solder pastes: there are a lot of different
formulations for solder pastes. The products you are currently
purchasing through Rio Grande are designed to be 'restrictive'
solder pastes. That means that the solder paste will not spread out
as you heat it, and will tend to keep the solder alloy right where it
is placed on the assembly. This solder paste can also be used to
solder on vertical surfaces, where the solder paste won't drip. An
added benefit of the restrictive solder paste is that it can help
hold small pieces in place during soldering. Restrictive pastes do
tend to dry out over time, because the ingredients that make it
restrictive tend to evaporate more quickly.
We also produce nondrying pastes which are not restrictive. These
pastes are a better choice if you desire a long shelf life, or if
you prefer a paste that will tend to spread out during the early
stages of heating.
Send me an email off list and I'd be happy to send you some thinner
for the paste you currently have, or a sample of a nondrying paste if
that is your preference.
Manufactures of Brazing and Soldering Products for Gold, Silver,
Platinum, Palladium and nonprecious metals in Paste, Powder, Sheet,
Strip, Wire, Chopped and Sphere forms.