Main tips for postrender work: 1. Duplicate your first layer. This saves your original render from being destroyed by an 'oops'. 2. Render to Tiff format so you can have a mask for working 'behind' your figures. 3. Make friends with the eyedropper tool to pick up color from the picture for laying down new strokes. 4. Make friends with the smear, blend and dodge-and-burn tools for altering existing parts of the pic. 5. Be sparing of filter use, but know that they can help in a big way at times. 6. A 'painterly' approach often -lacks- the level of detail that a 3D render makes automatically. Decide where the visual focus will be, and be ready to 'sacrifice' other sections of the pic.
my two cents worth

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."