Is there any way to reconstitute dried up paste solder?
Just mix it with some distilled water and it should go back to a paste. It might not flow as well as it should though.
I would recommend using either liquid but life or mineral spirits. Mix in a few drops at a time until you get a smooth spreadable consistency. I have used the liquid bur life with great success. Both work very well
Distilled water. I have this problem also, works well with this.