Painting is 12inx12in, room is about 17ft square
Small painting, big room. Why 55mm? 120 would be fine and being macro gives you much better manual focus control
I took some more shots without using the gray card. I spot metered off several locations in the painting (the sky, the grass, the road,etc). Obviously, I got some very different readings at each location.
Exactly. This is why a camera meter is useless without a reference such as a grey card. Either that or you use an incident meter which measures the light arriving at the subject, not reflected off it.
I have aperature set at f/8, 100 ISO and I averaged the metering to where the brighter sky read 1/3 stop over exposed and the darker grass and road was 1/3 to 2/3 underexposed. I think the shot has much better exposure.
You are all over the shop. There is no place for "average"
But, I have a question still about gray card use. It would seem that the gray card I was using has a higher reflectivity than the painting and therefore the exposure for the gray card was underexposed for the painting. So, since very few images have 18% reflectivity, what does a gray card actually do for you?
No, it was correctly exposing the painting. If the artist wanted the painting bright then they would have used bright colours. The job in art reproduction is to reproduce, not improve. You don't change a Rembrandt into a Monet.
And another question about using the colorchecker. I created a colorchecker profile in the exact light I was shooting the painting in. If I shoot another painting of the same size such that I don't have to move camera or lights, can I use the same profile? Is there any reason to shoot the colorchecker in front of each painting I shoot? Those of you that use a colorchecker or similar what exactly is the workflow procedure and what does it do for you? I can definitely tell a difference between Adobe Standard and the custom profile (the custom profile seems to match the painting better).
No, if everything is exactly the same light and camera then once should be fine. Note that the colourcheckr can do the same as the grey card. You can set the exposure with the white. The colorcheckr is for colour accuracy. You need to fix the exposure in post anyway as the camera response is not the same as the eye. i.e., you need to make the whites white.