I am stuck on disagreeing with the example because it's not a valid one. Wood by nature is porous, mdf is extremely hygroscopic. Most quality wood glues now a days are water based ( notice they always tell you to keep a damp cloth handy to wipe up excess //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif ) By the time you apply the glue properly to both pieces being bonded together, put them together and clamp them. Glue has already started seeping into the wood. Making clamping them to tight null and void.
what do I think ? Could be a number of things.
Bad glue, it happens every once in a while that a crappy batch makes it out. Or the glue had been sitting for a while.
Could be user error, I have seen that a number of times as well. Could be not clamping the joint well enough or allowing it to sit long enough.
But most of the time barring human error, it is caused by a foreign matter on the surface of the wood itself not allowing the capilary action of the glue to take place. Main reason i give a quick once over with some sand paper before I glue anything. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif