Uneven Polarization with Wide Angle Lenses
Polarizers are one of the most commonly used filters. They can darken the blue of the sky and help remove or reduce reflections from non-metallic objects. On "normal" or telephoto lenses they can perform this job quite well, but on extreme wide angle lenses you can run into problems. Often, when people first try to use a polarizer on such a lens, they wonder how to solve this problem.
The "problem" (actually "effect") is caused by the basic physics of light and is inherent to all polarizers. Polarization will be at its maximum when facing 90 degrees to the light source (typically the sun), and at a minimum at 180 degrees (directly away from) or straight into the light source. Wide angle lenses though see a lot. The angle of view at 18mm in the horizontal (long) dimension is 90 degrees so no matter which way you point such a lens, one side of the frame will be much more polarized than the other.
Moderate wide angle lenses are less of a issue since they have narrower angles of view. Beyond about 35mm you should not run into any "problems." With care, you can effectively use a polarizer at wider angles of course, so long as you pay attention to what direction you point your lens. Wider than around 28mm though and this uneven polarization is all but unavoidable.
These days, you can always correct such "problems" after the fact in Photoshop of course, but somehow it's always more satisfying when you can create the image in camera. You can't fight physics though so iI'm afraid your options are somewhat limited.