I find even at 18 mm I get distortion and often better results stitching images together where you can get full 360 degs. I am told Hugin is the software to use but it will not run on my PC but is free. There are also fish eye filters that go on the end of your lens worth a thought.

Polarising filters can mess up the focus on some cameras normal is to use a sandwich filter with polarising and 1/4 wave the latter turns it back so the focus will still work. They seem to be called circular polarising filters not a clue why as they are not circular polarising. You need to turn a linear polarising filter and in some cases the lens hood stops one getting at the filter so you may also need a new lens hood.

The polarising filter can do odd things when something else interacts. Any LED display is likely to show blue blotches when take with polarising filter on and it also reduces light. So a sky light filter is some times called for instead.

Two polarising filters turned against each other will act as a neutral density filter but not if there is a 1/4 wave filter between them. This is the test used to see if just polarising or is sandwich type.