This is a good idea as well, a lot of people forget about corrosion protection AFTER the repair is done. I made a wand that attaches to a binks #7 that I can spray epoxy into all the nooks... I just let it run out... plastic under the car.
I'd really like to see your wand and how you made it. I've been thinking about how to treat the insides of panels after I sandblast what I can get at. Eastwood has an aerosol with hose and nozzle that sprays in a circle, but I'm not sure its the product I want to use. http://www.eastwood.com/internal-frame-coating-w-spray-nozzle.html
Your advice for sanding filler is really great. I've done just enough of it to understand very well why the methods you describe will make a big difference. Shaping the filler can be very frustrating and you've explained it well.
I've read a great deal on the various rust treatments. Lets just say there is a lot of opinion out there and sparse hard data.
I've also been researching the new rust removal products that work by chelation. These are highly impressive. It's worth looking at the web sites to see how they are being used even on complete bodies with a spray and collect/recirculate method.
