I got lots of rejections of photos shot with circular polarizer. Inspectors claim that it was over filtered or had wrong WB.

Rejections have nothing to do with polarizers. There's no such thing as "polarizer generated rejection".
Landscapes + blue sky / water / reflections means you need a polarizer.

That's not necessarily an issue with the polarizer although it will probably make it worse. It looks to me as if you're trying to shoot a white building in bright sunshine and the camera is exposing for the building leaving the sky underexposed.

A couple of years back such images would often be accepted but to be honest they rarely sold anyway __ the black unnatural-looking skies hugely reduce the commercial potential of the image.

Hm , I would first search for some landscapes on IS sorted by downloads before stating that unnatural skies are reducing image potential cause the results will
show just the opposite.

Obviously it is dependent on the subject matter and the images in question aren't landscapes. From my own experience near-black skies appear to have a negative impact on sales of architectural images.

The OP asked a question and I've taken the time to advise on the issue from my experience. If you've made it your job here to nit-pick and argue about irrelevant detail then I won't bother in future.

melastmohican,

I saw the first image in 123RF.  I don't know if it's valid to make a judgement, as it seems to me that 123RF resaves our uploads with a higher degree of compression, but when zooming to aprox. 100% I can see some banding noise in the sky and reflections.  This can be already present in the original, but sometimes I also observe them being intensified with edition, so maybe this is what they are complaining about, not the polarization.

Regards,
Adelaide

I don't see a problem with WB, but "overfiltering" means, as I understand, problems with excessive edition. What I see is not noise as we normally see it, but banding, which can be present in the original, but not an issue, and then be made more noticeable after the edition process.

I found a better technical explanation here:

In 8-bit image formats, such as JPEG, each colour number is specified as being in the range from 0 to 255. If you apply something like a curve to such a pixel the curve might say "darken the colour by 43.41%. If the original colour happens to be, say, 133, then the new colour will be 133 x 43.41 = 57.7353. But only whole numbers are allowed so the new colour will be rounded to 58.
In a series of such operations the rounding errors can accumulate, resulting in what is called posterization or banding. This looks like streaks of colour instead of smooth transitions and is often seen in sky. Banding is simply the most obvious manifestation of the problem; accumulated rounding errors affect everything.

Regards,
Adelaide

There are several problems connected do this "CPL issue" - its not necessarilly true that dark skies have negative impact on sales. Most great landscapes and architecture shots are done with CPL, same with flowing water etc. Nothing to do with "unnatural" look - those oversaturated and overcontrasted pics widely present at all microstock sites are "natural"? HDRs or IRs are "natural"? And those shadeless studio shots or swapped (sandwich) skies at many landscapes are "natural"? Dont think so...

The problem is that some editors simply hate CPL and so reject, Im suspicious some of those even dont know how CPL works and think you "overfiltered" that in postprocess. You must realize there are many folks amongst microstock reviewers often hired from contributors - many of those do only studio work or even illustartions and have NO idea about anything like HDR, landscape filters or real world print. Some of stated "artifacts, noise or banding" issues are also result of non-calibrated and/or cheap super-overcontrasted LCD monitors.

Another issue is your real shot - shooting white building and blue sky with spot metering on white results in seriously underexposed areas in the sky because its far beyond the capabilities of sensor dynamic range. That means there is real noise even at iso 100. If you push levels too much in postprocess or push too much on noise reduction it results in typical blocky artifacts and banding. Its not necessarilly always this way but its typical technical problem.