I've also recently done some work on a poster, again for a friend. She sent it away to be framed, and when it came back from the shop it had been nastily creased, to the extent that it had white bits showing through the printing:
Before
I flattened the creases out, touched up the white bits with watercolours (I'm particularly pleased with the colour match on the skin and hair, which was a sod!) and then mounted it again, properly this time:
After flattening
After touching up
Remounted