painting with light - muti exposers

Any help? I have been trying to build a "painted with light" photo with multiple 8 sec shots with my G1 and Photoshop. My current approach is by using my base shot then layering a shot with light added then applying the deference mode to the layers taking what is left then building the final image out of many of those. I think this is what in After Effects is called a "difference matte". The only problem is that I'm having problems pulling off an adding my many shots for my final image. I've used the layers matting command remove black matte with some success. Has any one been able to do anything like this? Or has any one come up with a way to create multi exposer shots that have the same quality as film shot this way. Let me be the first to say that yes this can be done quite easily with a film camera or a camera with longer exposer times, but I would like to this with my G1, any help you can give would be greatly appreciated.

You might look on the Internet for a freeware program called ASTROSTACK.

This was developed for astronomy purposes and allows multiple exposures to be aligned into one sharp one.

I have tested it a few times and it works, although it will not build up a fully exposed image from a number of underexposed ones as you might want. I think it is mainly for removing atmospheric effects such star
twinkle.