It was my birthday yesterday and my boyfriend was coming over to do something fun. I wasn't sure what I wanted to do and he suggested the night before... undercoat my dollhouse, something that I've been wanting to do for years now.

See why I love him? So, we spent ages moving things around in my house so some rooms are free to paint, and yesterday we started.
However, I'm now noticing that it's not at all an even job and how hard it is/was to get paint into the corners. How on earth do I fix this? We started using rollers but they don't go into the corners so then I was using a paint brush to get into them (since the corner brush I'd been given is so obviously for full sized rooms and won't go into the corners properly). That looked really bad. So, today when I was by myself I experimented with a sponge on a handle, and I did the corners first and then rolled the rest of the room. There are still 'lines' and patchiness though I tried to be as even as possible and what I did with the sponge shows on the corners.
How on earth do you get it right? It's just undercoating but since it's white I was hoping I could use it as whitewash or white ceilings in some of the rooms.