Are you talking about the banding? That's a common artefact of 8-bit per channel colour. Try kicking your project up to 16 bit or 32 bits.
Some useful blurb from the manual.
Alternatively, adding a bit of noise can also help break up the banding.
You never said which formats you rendered to. Could also be a limitation of your export codec/compression settings.
All the best,
Ben
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