Originally Posted by Mitchman
Working body panels is an art, no doubt about it. The best advise I received came from the fellow who taught me to work with lead and Bondo in 1973 (H.S. auto shop). He said as soon as you find there is a need to add filler to a panel, you become a sculptor. Once you realize that, you will also need to realize that you are no longer shaping the panel, you are sculpting the fill.
In other words, once you spread fill, forget the panel until you are happy with the shape of the fill, or when you *JUST* see the panel again as it's high spots poke through. At that point you have a decision to make. Either stop or add fill to the low spots and sculpt some more.
Most of the trick to sculpting anything is knowing when to stop removing material.
Very true, that is NUMBER ONE the absolute NUMBER ONE mistake guys make, taking off too much. Newbes very seldom leave too much, they always take off too much. That edge of the filler should literally be transparent, it should transition from filler color getting thinner and thinner like a fading color to the surrounding metal. VERY seldom does it go from filler to metal with a definite line like in these photos and still be smooth. It does happen under certain conditions, but it is VERY seldom.
Henry (Tech69) has some great videos but I don't know where to find this particular lesson so I grabbed one off the net. This guy has some great explanation of the process. Click on the second video down on the page. I wanted to post a photo here showing the proper look that blocked filler should look. I googled images and found this image but it ended up being a video, which is cool, the video is very good. I don't like using the course paper as long has he does but other than that it is some great basic info. I also don't like how close the paint feather edge is to the work but there are lots of different ways to pull it off. You will notice he warns of hitting the filler when you feather edge the paint away after the repair, well if you feathered it further to begin with there wouldn't be a problem. But check it out, it's a darn good explanation of the process.
Body Filler - How To Block Sand Bondo On A Auto Repair