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,,