Can the Blue Fill be used to glue ply doublers or motor mounts in place?
Or joining foam wing halves in place with a balsa root rib in each side?
How about gluing ply bellcrank mounts in place?
I'd like to know if this Blue Fill has gluing strength?
It is more than adequate for your application. For everyone else, probably not.
It has some bulk strength, and is reasonably strong as an adhesive, but it is not appropriate nor intended for gluing in small bond lines and has minimal penetration into the material of the joints. Make a 1/2 thick plate out of it, and you would have trouble breaking it with your bare hands. Not as strong as Epoxolite, but also much lighter. Don't use Epoxolite for these applications, either.
It is not "Blue Fill". It is Poly Fiber Super Fil:
https://www.aircraftspruce.com/catalog/cmpages/superfil.php?clickkey=12902 As the name suggests, it is intended as a *filler* for composite construction on full-scale aircraft. It doesn't work too well as a filler on balsa, because it is much harder than wood. But (Howard's problem aside) it does work very well for fillets on model airplanes, and is admirably light, easy to sand compared to the alternatives, and paint sticks to it well.
I am not sure what you mean about "balsa root ribs on foam wings". You can put that together with Ambroid for what little good it does you, but foam wings do not typically have "balsa root ribs" and count on only the surface joint, the overlying fiberglass and epoxy reinforcement strip, and the joint at the landing gear spar (if any).
For any solid structural connection, you *do not want filler* as a general rule, you want the material touching with a minimal bond line thickness. If you have gaps, and you absolutely must use a structural filler, then use something like milled fiberglass powder and a proper adhesive.
Brett
p.s. Note that the "you don't want filler" extends to liquid fillers, at least for maximum strength. Almost all consumer epoxies are heavily "filled" with inert resins or oils to get the mix ratio near 1:1 or 2:1. That significantly reduces the strength from the maximum possible. Fortunately, it doesn't matter, because for almost any purpose we might use it for, it is still far stronger than you need it to be.