Touch up of an old painting

I painting this Geisha about three years ago. So I know that the painting is dry.
The painting has never varnished the painting. As you can see I want to touch up two areas that I left badly done. 

My question is; Can I just paint over the dry paint or do I need to somehow prepare the areas for touch up.
If a prep is needed what is the prep procedure?

Thanks a bunch


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