Paint.NET :: Un-erasing Part Of A Layer?

In Photoshop you can erase part of an image with the color white with the paint brush and bring it back with the black brush. I used the erase tool to remove part of an image but then wanted to bring the edges back, but with precision detail, however i don't see a way to un-erase the edges like you can in photoshop. Example I have one shot of a sky and one shot of the mountains. I layer the two images and I erase the sky in the mountain image so the sky image in the layer below comes through. However, the divide between the mountain photo and the sky photo has jagged edges and I want to paint part of the image back in with precision along the rough mountain edges.