Electronic – Is thinner solder better

I've left wet paste on a board overnight in cool weather without problems, but I'd be worried it would dry out over a few days - especially if left uncovered in a refrigerator (although I'm not sure if this would adversely effect the reflowing).

If there are pads under the parts, then soldering with an iron is out of the question, but a hot air rework gun should work fine. Depending on the complexity and the number of missing parts, I would either:

• Flow now and solder the additional parts later (you can place a cube of metal under specific parts of the board to reflow individual parts on the hotplate).
• Clean the board and restart when you've got the new parts

A hobbyist isn't constrained by rules like ROHS or anything. Your only constraints are your soldering iron, and the availability of getting solder.

63/37 Tin/Lead Rosin Core is a relatively low temperature, easy to use, easy to find solder commonly used by hobbyists. Choose the size depending on what you are going to solder (the bigger the parts, the larger the solder you want). 60/40 Tin/Lead is also commonly used. You can try 62/36/2 Tin/Lead/Silver if you want to get fancy.

Best advice, don't sweat the small stuff :D