Sanding is the art
The art of body work, what did I say? You read right, Sanding is the key and Sanding is the art. I have long been a mechanic and stayed away from bodywork for the same reasons most others do. I thought it was messy and just plain cantankerous generally speaking. But and there is always a but, I got into a truck project for my better half and started with a rusty cab, bed, and doors that I had to actually build from 3 doors. Sounds like a load of body work and you are correct, so here goes and the point of this rant is to hopefully give other beginners a leg up on the theory and application of Sanding.
Over sanding is a common problem and the filler makers love you for it. The purpose of sanding is 3 fold, first is shaping, second is blending, and third and final is smoothing. If you get this order out of order you are wasting time, paper and filler. Do not get me wrong here I have made all the mistakes mention. I will start with shaping; the best tool I find for shaping is some sort of mechanical device. Da, air file that sort of thing, this will allow the user to get a ruffed in contour, a flat panel also has a contour, it is just flat. Now that a shape has been established you run your hand over that area and find it a little high. At this point I turn to a block and with medium to light pressure I work the area some more. I work on the area until I think it appears and feels flat. I use the word appears because that area is still not quite there. When looking at the area there are the outer edges of the filler application that one can see as a defined line between the body (and sometimes another layer of filler), this line is higher then the under area. This is where the art comes to light; this little demon must be blended. Blending this area is best done by hand and light pressure. My way of dealing with this is to get a small block 3x1 inches or so and go around this define line. Sanding in an outward direction from the filler to the line. Sand until it looks like an ocean wave that washes ashore
WHAT? No defined line, blended. A line that runs onto and into the lower layer as if the filler just runs on until it gets so thinned out it disappears. At this point the center area of the filler will be a little higher then the edges just blended. They way around this is to go back and use a long block or the old school paint stick and smooth out the entire area until the complete area has become one.
Hope this put some light and added food for thought that can be of use to the boots of this hobby. The pros know the word blending cause they do it all the time. That is what bodywork is all about, blending old with new, wrinkled with smooth, whatever you want to call it