Nothing is unglueable. You just need the right glue.
Now there's a bold statement. There are a lot of "things" that can't be glued. Now if you stated there is no solid material that cannot be glued.....
Getting into the existential realm here, but I think in a sense
@cwbullet remains correct. Liquids, gases, and plasma are all made of elements, and all are amenable to some sort of cohesion, whether that be gravity, ionic or covalent bonds, or other "forces" which are beyond my knowledge. Of course, if you apply some of those forces to the elemental or molecular levels of these liquids, gases, and plasmas, after application theymay no longer BE in the form of liquids, gases, and plasmas.
I am guessing the gas clouds in the Universe are largely held together by their own very weak gravity. I'd blame it on Gary Byrum and der MicroMeister, but those clouds predated them, so no-go there. Those two are just up there "gluing" those rockets you launch that never come down to the heavenly realms. I imagine they have quite the collection there.
In the semantic sense, because the "common man" definition of a good glue joint is generally along the lines of "the two components are
solidly glued together", a good argument is that applying the term to NON-solids is a non sequitur to begin with.
Getting back to "real world rocketry", there is another "modifier" that needs to go on the statement.
"Anything solid CAN be glued, but there are things that are not easily glued WELL."
Sort of like the F4 Phantom Aircraft was often described as proof that, "If you put big enough engines on it, anything can be made to fly."
Or maybe that was
@Daddyisabar 's quote, I believe rephrased as, "Trust in Thrust."
Another concept that has been largely ignored is that there are other ways of attaching two solid components than glue. Bolts, rivets, string/floss, "tongue in groove" joints, elastic bands, friction (that's how most of my motors are held in place with the motor mounts, except the rockets I fly that HAVE no motor mounts
and fly quite nicely.) I believe a number of Estes models use plastic fins mounted in grooves in the fin can, I can't remember if you are supposed to glue them in place after insertion or not.
@jqavins made an interesting remark about using a second substance, such as metal, as an "in-between" joint, sort of like, if B is metal, and A will not easily stick to C, but both A and C stick to B (metal), put metal between them. For a long narrow joint such as a fin root, direct placement of a strip of metal between the tube and the fin material may or may not work, and the purists may argue that metal in particular is a violation of the rocket safety code (to which I say, how about motor hooks, screw eyes, and electronics? but I digress.)
There is at least one more option, however: solid fillets. Say A doesn't stick to C, but some other solid and relatively strong material sticks at least partially will to either A and/or C. Say the Body tube is material A, the fillet B, the fin C. If you make fillets the length of the fin root of material B (or whatever else you are gluing, ASSUMING a straight edge), you can glue, sew, screw, or bolt those onto the body tube. Dimensions are the length of the fin tube by, say 1/8" x 1/8" for low power, could go larger for MPR or even HPR (and the bolting or screwing in place would be easier with larger rockets. "Sewing" them in say with dental floss and a curved needle is do-able but would be challenging for an outer surface mounted fin, easy-peasy for a through the wall fin provided you don't mind you fin slots going to the base of the tube and sliding it in place.)
Anyway, once you have the fillets in place, you can put Glue the fin made of C in between your pre-placed fillets made of B with some compatible adhesive that works for B-C joints, whether or not it works for A-C doesn't matter.
Advantages:
Assuming you can find a way to ATTACH (adhesive, screws, bolts, string, whatever) B to A, and assuming you have an adhesive that works from B-C (by the way, if you bolt or screw the fillets into A, you can even MAKE them out of material C, although there are some materials that don't even stick to THEMSELVES well, so a successful C-C adhesive joint is not guaranteed)-----anyway, gives you an indirect way of fixing Fin C to body tube A.
Because the fillets are relatively small and light, they are generally very easy to glue onto A in perfect alignment, you don't have to hold them very long before they "tack". The double glue joint works well here. So your fin alignment tends to be pretty good.
Provides a much greater surface area for the joint, thus generally a stronger joint. Note you don't need a lot of glue if you put space the fillets right, the fin should initially basically friction fit into the groove without glue before you even start.
Often provides a faster "tack" when fin is placed, likely because of the larger surface area. Again, double glue joint works well.
Disadvantages:
More complex. Time-wise however not necessarily longer, the fillets go on pretty easily and quickly, but you do need to let them dry completely before attaching the fin.
Adds a bit of weight.
Adds a bit of drag. This may partially be overcome depending on the thickness of your overlying fillet (I assume you lay another GLUE fillet over the solid filled.) a "thick" glue fillet may completely cover the solid fillet.
Again, we are thinking different materials. I can say from experience this works VERY well even for compatible materials like regular cardboard body tubes and balsa fins, it is how I make most of my standard fin rockets now. I have yet to have a joint failure, although I have had the BODY tube rip off or the FIN break, but that just proves how strong the joints are. In this case however, I usually only use ONE fillet, I stick it on my line marker, one for each fin, let it dry, then put the fin on. I could add another fillet on the other side, but I rarely bother. Then again, my scratch build goals are mainly engineering challanges, I like to see what works, not all that concerned about aesthetics.
More than one way to skin a cat.