Texture painting - Erase Alpha doesn't save?

Hi, I’m very new to Blender and I’m having a spot of bother with materials and texturing. I’d like to paint rust on to an object that already has a texture applied.

This is the approach I used…
So, I started my project in Blender Render (v2.78), created a material with two slots - one with the base layer, one with the top (rust) layer. Using the Erase Alpha I removed some rust from the top layer and it looked great! (well, good enough). Then I saved the images as they had a * by them, but when I reopened the file, the Erase Alpha detail had disappeared.

Where am I going wrong?
Is there a better method?
Is there a tutorial? (I found one for cycles with a jerry can, but I couldn’t figure out how to translate that to Blender Render, but that’s probably because I’m new to all this!).

Any help would be much appreciated.