The issues you are having sound as if your Polarizing filter is giving you color casts and possibly the digital sensor is giving your some problems.

Over the years, a lot of copy work for reproduction and entries into shows for artists. Never had an issue with the 8×10 and 4×5 and 35mm chromes. Now with digital things go faster and feedback is immediate.

A very good, sturdy tripod is something many seem to overlook. A lot of “sturdy” tripods do introduce minor movement when a shutter is tripped. Damping with a small bag of rice over the camera body while bracing legs with a sandbag/vibratio absorbing weight helps here. And no center column use – as it magnifies movement. I’m doing exposures of a few seconds to a few minutes and movement is a killer, but you are most likely well aware of that.

Light color temperature needs to be checked regularly – even with digital. Then the age and condition of the polarizing sheet used for the lighting. On the Polarizer over the lens I have a nice, neutral Nikkor filter I use. Second choice for me would be the Singh Ray filter. Both I have had for some time and know they work.

With digital you may need to check more than one camera and lens combination side by side. As silly as it sounds I have found my best copy work for art is with a Sigma DP3Merrill camera. Slow and clunky – like a mini View camera. But the sharpness and the color! Another body I found very good is the Canon EOS 1DsMkIII. It was over $8,000 new but on the used market is under $1000 now. These two have proven to be good for copy work with various types of art. Low ISO, optimum lens aperture. With the Canon I use a Macro lens for its flat field and sharpness. The DP3 Merrill is a fixed lens body. Sure nice to have no worries about dust on the sensor. I do use Color separation guides as well as Gray scale step wedges and a good ruler alongside the artwork for a frame as well as one without it. This helps with printers if and when the images go to press.

Work in a room that is dark and maybe with the camera centered in a large flat panel that is painted flat black – as this helps a lot to control some reflections. Any bright surfaces are masked with Gaffer tape so I don’t get a light stand putting an odd reflection somewhere. I do use hot lights. Some friends like strobes. Just personal preference.

Below is a link to a good black that cuts reflections. It works well.
https://culturehustle.com/products/black-v1-0-beta-the-world-s-mattest-flattest-blackest-art-material

I would change one thing at a time. If you do more you introduce added variables into the picture – literally as well as figuratively.

Good luck, let us know how things go. I am on here from time to time and find some interesting things – and a lot way over my head. I photograph. News and commercial and personal Art since the early 1970’s.

I have done this for a living since early 1980’s. I no longer look for work, but I have a whole lot of experience. I ended using a Betterlight scanning back with HID Northlights. There are photographers all over the states with this kind of equipment and knowledge.

Jim, where are you located. Maybe there is someone better in your area.
For super exacting work there is a number of fiddly steps. Just hiring a pro photographer is not a solution, unless they specialize in fine art reproduction work.

Gear you will need if you want to do this yourself:
tripod
a geared tripod head [allows for precise independent movements]
continuous lighting & polarizers for both camera and for lens
Zigalign mirrors [helps to perfectly align the camera to the copy board : note most pros don’t even know what this is]
a larger than your painting clean white card [used to establish light and lens fall off]
Equalight software [is used to determine every pixel exposure and then applies that math to the file of your painting copy and the Macbeth color chart]
Equal light color chart.
A profile making software

Crossed polarization does work, however, the camera filter needs to be backed off slightly, and the camera profile needs to be made at the polarizer setting.

Sometimes I layered a non cross polarized image with the crossed polarized image. Then I erased away and kept the good parts. This is for experts.

If your pro photographer is worth his/her salt, they will do the job over for free or at a reduced rate. They will work until they get it right.

They need to be able to print samples on the premises with you sitting beside them.

Good luck.

Thank you so much for your lovely ideas. I really appreciate this