You could mix some bakers fluid in with it. Though the problem with thinning is you will "dilute" the solder content and may not get enough to seal the rivits.

I would probably tin the inside in strips first and then fit the rivits before using a small artists brush to apply a bit of the paste to the back of each rivit rather than paste the whole area.

I seem to remember last time my solder paste had gone rock hard, I broke it up and crushed into a powder using a pestel and mortar, then added a bit of water to a small amount of it to mix it into a paste again. I have kept most of it as a powder so I can mix it as required in future.

It seems to work just the same as it always has done using this method.

You do need to make sure it is stored in an airtight container so it does not dry out. I use old 35mm film containers for that purpose.

Phil

I would buy a syringe of easy silver solder paste from a jewellery supplier such as cooksongold.com. Unlike the poor service of another well known jewellery supplier criticised on here recently; their customer service and delivery times have always been spo-on when I have ordered from them.

Jewellery silver solder paste is much 'softer' than Fryolux type soft-solder paint and very controllable in application (its primarily used to repair very fine jewellery where there is a risk of either melting the item or there oo difficult to control 'normal' silver solder pallions

It can have porosity issues so whilst I wouldn't recommend it for 'structural' situations like caulking, it should be perfect for your rivets