c) use a small artist's brush to help retain and saturate the entire opening with cyanoacrylate(super glue), the more it soaks in, the better. It tends to want to easily soak into where the water originally got in and carried away or corrupted the original resin of the MDF. Wood glue and other adhesives won't penetrate deeply enough and many times just cause a new jagged and ugly crack to open up right next to the original.
d) work fast and tighten as many clamps as necessary to bring the corner back to it's original dimensions being careful to blot up everything that squeezes out, leave it clamped at least 24 hours.
I've done some amazing repairs of particle board corner repairs this way when there is no missing material with about 80% being pretty much good as new afterward. Total outcome will depend how badly water soaked the cabinet was, and the type of materials involved.
Superglue has potential, as you can get it in varying thickness/viscosity- from water-thin, to almost the consistency of maple syrup.
The only caveat- and this is a BIG one- is that once it's on there, it AIN'T going away. It's hard to wipe down, as the pressure of wiping it can sometimes cause it to start curing! You have to be fast, and have a delicate touch with this. Also, make ABSOLUTELY SURE you have everything in EXACTLY the right place before application, because you will NOT get a second chance.
The nice thing is, though, is that once you have it in place, you can instantly cure it with activator/accelerator spray (which will also be available from whatever woodworking store has the superglue you will need). It will take a small interval for the cure to propagate throughout the joint (usually a minute is plenty), but it's way faster than waiting for epoxy, or more abjectly, wood glue...
I've done some stuff with superglue, that afterwards, I was almost in disbelief that I could do. If you sand it, it's even actually stainable (I've actually "faked" wood by using thin layers of superglue to build up and fill a chip-out on an edge- apply a bit, sand it and shape it, stain it, apply more, sand/shape it, stain it, and so on). If you get the hang of it, it can work well in replacing material in a chipped out edge. The cool thing, is that since the superglue is transparent- as you sad and stain it, you get that translucent effect that you get from real wood (that iridescent thing where the wood changes color depending on the light direction). It looks a LOT better than opaque wood filler, IME!
I've gotten it to where it looked like, even under close scrutiny, that there wasn't even a chip in the first place, when I was lucky. This isn't always possible- but I can almost always get it to pass the "one foot test", when looking at it (it looks absolutely fine from a foot away)...
If you saw my Fisher X202 tube amp restoration pictures from before (the mahogany case)- that was a "survivor" that just needed to have loose parts (veneer edges) reattached and stabilized and some corner chips slightly reconstructed. All of that was done with superglue and some proper mahogany stain, believe it or not. The object was not to make it look brand new (I wanted to keep the character of the original finish over as much of it as possible- including some of the slight dings and such)- but it WAS to make it sturdy and handsome, where it would be presentable and usable without falling apart. And I think that happened, quite well enough.
Regards,
Gordon.