The goal with the panel edges is for them to be thin and consistent. In an ideal world, there would be no body filler on them, especially moving panels like doors. With the edges being as close to bare metal as possible, the chances of chips goes down dramatically, and the appearance is much better. When looking at a panel edge, it should look bare metal. If the door, for example, is a hemmed edge, that’s all it should look like after the vehicle is painted. If that edge gets filler built up on it, it will be thicker in places, giving away that it’s been body worked.
There is some room for error with hammed edges. While it would be nice to actually metal finish them, that is not realistic. A very close guess to where the edges need to be can be made with using the aluminum straight edges. But there are so many variables once the body work is actually done, that having edges that are completely metal isn’t very realistic. A small amount of body filler is acceptable, but metal should still be seen in the edge, and it should not get thick at all.
There are some vehicles, like earlier Lamborghini’s, where the door edges aren’t even hemmed. These edges are just the door skin edge and the door shell spot welded together. Situations like this, where the edges actually do need to remain sheet metal, require as much work as possible in making sure they are adjusted right. These edges may also require some adjusting as the body work begins, to make sure its actually sheet metal.
Besides making sure the edges still look like metal edges by not being thick from body filler, and the edges not chipping as easy when they are metal, the look of thick edges just isn’t good. It can make the vehicle look heavy and not quite as dialed in.
Almost always, the edges of the panels will be low when using the lengths of aluminum to check where they are. Sometimes, this is an easy fix, but sometimes it can be a fairly large project to get the edges where they need to be; just barely touching the aluminum as it’s spanned across the body.
Hopefully, all that’s required is the easier method. This is normally some hammer and dolly work to get the edge moved out lightly. Normally, this causes there to be a lower area in the panel just next to the edge. The more minute this low are is, the better. Another method for small adjustments is using a Knipex parallel jaw pliers. A piece of paint stick or something similar should be used between the jaw and the door skin to distribute the pressure. This method should be used with caution, and only for smaller adjustments, as it can easily cause damage.
Sometimes, if a lot of adjustment is needed, the door shell might need to be cut and moved out. In the same vein as this route, the problem might also be that the panel just has far too much crown in it. Sometimes, a shrinking disc can be used to shrink some of the crown out of the panel. Other times, a new door skin might be required. Deciding if there is too much crown or if the edge just needs to come up is partially a judgement call, relating to the surrounding area. If crown is taken out of that panel, and it makes it lower than all the other adjacent panels, the crown is not the problem, the edge is the problem. If that panel is higher than everything else, and you’ve done as much adjusting as possible, then there is too much crown in the panel.
The treatment for edges that are not hemmed edges, like most quarter panels, can be different. As with the door edge, in an ideal world, the edges would still be metal. The edges that are corners or flanges, like a quarter panel aren’t as delicate, because of being a corner instead of a sheet metal edge. With these corners, it’s also impossible to see if filler is on the edge, since it doesn’t appear thicker like a hemmed door edge would become thicker.