What DR1DRIVER said X2. I didn't mean to imply you can use fillers to mask shoddy building or big holes in wing spars, etc. We've all see the pilot that brings the model that is 90% epoxy. It's amazing they fly at all. Proper componet fit is critical to strong and light builds. However, there often comes a time (be it after a crash, or during a new build) when a minor flaw in structure exists that doesn't warrant a complete tear down/rebuild of the area. I.E., the part is attached, but there is a minor flaw in the joint. In such a case, a small fillet of epoxy/microfiber will often solve the issue and/or a gusset, doubler, etc. I haven't had many ARFs, but the ones I did have all required some "patching" out of box to meet my personal standards as some parts just didn't fit quite right IMO on the prebuilt parts when the ARF was assembled where ever it is that they assemble ARFs. Granted, a patch is heavier and probably not as strong as a properly fitting part, but it is easier than tearing the plane apart and rebuilding, esp a new in box ARF. Deciding when to compromise and patch is subjective and takes some experience. It is not worth losing a plane due to a patch. When in doubt, rebuild the area... that's what I meant by "If you must fill a gap in a structural joint." DR1 made a very good and important point. Others, Please do not read too much into my post, i.e. don't start slapping epoxy around on bad joint thinking you'll always be OK.

Cheers