Rene from Thunder Bay, Ontario has a corner joint in a counter top that doesn't sit perfectly flush and the water is getting in and swelling up the counter. What to do?
Unfortunately, if the counter has swollen up, you may not be able to fix the problem. However, to avoid problems when things don't line up but before the wood swells, you need to understand how the corner joint is held together.
As you can see there are probably three special pairs of slots under the counter along that 45 degree joint in the corner. There will be a bolt running across the joint and metal plates on each side. Turning the bolt or the nut will tighten the joint.
First, it is a good idea to seal the joint with silicone before putting it together. That will both help to glue the corner together and waterproof the slight crack.
Second, be careful with the tightening. It is all too easy to tighten that bolt so much that it breaks off the particle board that the brackets push against. Firm, but not too tight.
If your corner joint is not perfectly flat, but not yet swollen, you can loosen all three bolts and force the two parts of the counter to be level with each other. If you find that there is no glue or silicone between the two, open the joint up completely, apply silicone and then put it together. It will not slip again.