I use water.

Yeah, I use the wet finger technique. It just goes on pretty thick.
Makes very nice fillets though!

When the Super-Fil gets too thick in the containers, I apply a small amount of water and stir to a good consistency. I then mix how ever much I need. After I have applied the Super-Fil, I then wet my finger with water and proceed to smooth the fillet. I have been using this method for several years and it works great. Love this stuff, easy to sand.

personally I would never thin any two part system with water unless it was directed by the manufacture ( do as you see fit,, its your plane),, I throw mine in the micro for 10 seconds, just enough to feel it warm a touch,, then as was mentioned,, it becomes less viscous as you mix the two parts,, I started warming it when after sitting for a couple years ( yeah,, Wayne,, I know build more right),, it started to separate resin from filler,, warming it allowed me to re-blend the components and be more workable,, I resist adding solvents,, thinners or any viscosity modifying agents unless specified in user instructions,,

as to fillets not needing to be strong,, I am not sure I agree here,, the wing fuse joint,, and the stab joint are high stress areas,, weak fillet material can allow all sorts of nasties,, cracking,, and so on,,

I started warming it when after sitting for a couple years ( yeah,, Wayne,, I know build more right)...

 Actually Mark, you got me on that one. I've had my batch at least two or three years and it's not even half gone. I did use some recently though and it mixed up and worked just fine.