How do I clean the inside of my frame? Is it even necessary? I'm trying to fix up a bike that has been abandoned outside for a while. When I first turned it upside down, rust brown water started coming out of holes in frame around the rear stays.

I've fixed it up so the outside looks great, but I'm wondering if this standing water could've done much damage, and how I can clean and lube it up to keep it in good condition for as long as possible.
The most important thing is that there is no standing water inside the tubes. Usually there are drainage holes around the rear dropouts and bottom bracket. Make sure the holes exist and are not obstructed.

You can look inside the frame through the bottom bracket, seat tube and head tube. Some people use oil or wax-based surface treatments against rust, e.g. J.P. Weigle's Frame Saver.