First, I will assume that you have shot a few shots from on your tripod, using your CPL. Now that you have the shots, you want to combine them to remove at least part of the reflections in the glass and paint. The quickest and easiest way is through blending, using Photoshop to accomplish this task.
I’m going to suggest that you use the same shots that I used in this article: Circular Polarizers (CPL) and Reflections if you want to try the steps listed below to test.
First, you need to get all of the shots into Photoshop. Pick the one that has the least busy background as your master. I’m going to assume that you don’t have people or other moving objects in the background at this point. For each photo that you need to copy into your master, you need to “Select All” (Ctrl+a), then “Copy” (Ctrl+c), then go over to your master, and “Create a New Layer” (Shift+Ctrl+n), then “Paste” (Ctrl+v) into that new layer. Then, go to the top menu and move the mouse over “Layer”, then “Layer Style”, and select “Blending Options”. Change the blending mode to “Darken” reflections are lighter than the paint. Now, “Merge Down” the layers into one, by using the “Merge Down” command (Ctrl+e). Alternatively, you could use the “Flatten Image” command. Repeat for each shot that you made with the CPL. That’s it. It’s easy.
For white cars, and when you have objects moving in the background, such as people and cars, then you have to blend just the areas that you want to blend, while keeping the background the same as on the original master. This is a little more difficult, but not by much.
You start in the same way, but instead of just setting the entire layer to “Darken”, you first have to hide the layer, and then paint it onto the master. After you have pasted the image into a new layer, put your mouse over “Layer”, then “Layer Mask”, and select “Hide All”. This hides the top layer, and shows your master image. Now select the paint brush tool, and paint white over the areas that you want to have revealed on top of your master. If you make a mistake, you can simply “paint black” over the parts that you want to hide again. Once you have finished, merge down or flatten the layers, and go do the next image layer that you might need to do.
Once you have completed all layer, you can apply any touch-ups, light dodging or burning any parts that you need to. If you are going to edit it back in Lightroom, make sure that you export it as a TIFF file back to LR.