I've used a lot of Stits Poly-Spray silver on real aircraft many years ago.  It appears the Rand-O-Fill is simply a non-tautening Butyrate dope with silver as a UV protectant, same as the Poly-Spray silver.  I have never used the Randolph silver, though.  I assume that it is similar to the Poly-Spray in that it serves the same purpose.  Might make a good basecoat for filling and smoothing minor imperfections before the colors.  Or you could simply use it as a silver color.  I also expect it is high solids and will add weight quickly.

But no personal experience with it, so I probably shouldn't have replied at all.  But since I typed, it, it's as easy to hit "post" as it is to delete it.

Well, except for the price, there's no downside to trying it.  And none of us might ever know until you try.  I think you would find it useful to have around if you build a lot, anyway.  I used the Stits silver on a few model planes as a kid when my dad had some around.  None of those planes lasted long enough for me to actually tell if it were fuel proof or not, though.