The last couple of days has had me grinding, sanding, rust treating and glazing. Lather, rinse, repeat.... It is truly amazing how many tiny stone nicks kicked up from the front tires have pitted the doors. As my process of stripping this car continues the "bondo" areas are showing up. I've used it before, but it's like a sponge! The stuff just attracts moisture and in a year or so the area around the patch is rusted out. My method is grind and cut then reweld new steel where you can. Otherwise get replacement panels where the damage just isn't worth spending the time.
So, with that being said. The approach to this restoration rust issue is as follows:
1) grind all rust spots and blistered areas down to bare metal using the air grinding tool;
2) clean off the area with a dry rag;
3) treat area with rust converter (which I trust about as far as I can throw the car);
4) restrip the area to remove any trace about of rust possible and retreat;
5) prime with self-etching primer;
6) resand area and reprime with sandable primer coats;
7) resand to bring out any imperfections in the treated area;
8) use a spare amount of glazing putty to build up the spot;
9) resand and reprime;
10) resand and reglaze as needed;
11) prime smooth areas with new sandable primer;
12) wet sand to remove any defects;
13) spray with a base coat of the preferred new color;
14) perform steps 1-13 on ENTIRE car prior to shipping to paint shop for full repainting.
I didn't mention the cutting out of rotted panels/steeel and welding of new metal, or the other non-body related issues. As far as using those "fillers" for fixing holes, I avoid them like the plauge. I'd rather use steel, grind the flanged edges and use just enough filler to blend the seam. The less the better.
You can find panels at Walloth n. Nesch a fantastic supplier in Germany. You pay about 50% of the cost of parts for shipping, but they have complete panels and parts you can not find anywhere else.
http://www.wallothnesch.com/My order consists of front wing panels (bondo victims), rocker panels (salt victims), and parts for my euro bumpers (YES!).
I know some restorers HATE anything to do with rust, but I found that if you have time and are very thorough you can derust a car, or at least go far enough to delay further corrosion until you can replace or trace those hard-to-get spots.