When you have a large flat panel that is flexing the first thing you need to do is find out why. Sometimes you can stop it, other times you can’t. But if you can stop it, you’ll have a much easier time with the body filler work.
Hoods, decklids, and the roof are particularly difficult because the heat and weight of the plastic filler can have an effect on the metal. The good news is many times it is very easy to repair. First off, there is no such thing as a “flat” panel. All panels that appear flat actually have a slight crown or gentle bow up in the middle. Go to a flat panel and lay a straight edge across it. You will see that the straight edge in not touching the panel at the on the outer ends. If the panel were perfectly flat it would appear to the eye to be concave. It would also have no “body” and flex very easily. This is the problem with your large flexing panel; it has “lost” its crown and is now weak and flexible. The first place to start your search for a culprit is under the panel. Many panels have inner structure that supports the outer skin. When the outer panel has been damaged the inner structure was bent down along with the outer. This inner structure can be in the form of just a simple inch or so wide support running across the panel to the complete support by a stamped panel that goes covers the underside of the panel. These full inner structures can commonly be found on hoods and decklids. The inner structure can sometimes be bent down, causing your flexing. It usually is very close to the outer skin, with just a thin layer of a foam or urethane adhesive. It may have small “dollops” of this foam or adhesive that has been squished between the inner structure and outer skin or even a thin piece of tarpaper. You can push up on these low spots to return it to supporting the outer skin, as it should. But it is difficult because you can’t push it past where it needs to be. On this particular type of damage, the inner structure would need to go past the correct shape and then “relax” back down to where it belongs. It can’t do this of course because the outer panel is there and limits the inner structure from going up where it needs to go. Just as with looking at the “big picture” when you look at any dent, you need to search for a kink or bend that is holding the inner structure down in that area. If you apply pressure up on the low area and tap out these kinks, you may get it to stay back in shape. If these methods don’t get it back up to support the outer panel properly, you will need to “shim” between the two panels to get the outer panel up where it belongs. This can be done with sheet of tarpaper or more adhesive.
As a last resort a thin piece of wood like a paint stirring stick can be used. Of course, this is a little on the funky side but if you are haven’t been able to correct the problem, something has to be done. What you have to watch out for is applying too much pressure in one area. If you were to force a piece of wood in there, you will likely be making a high spot on the outside. That would just give you in a whole new problem. Sight down the body lines that are nearest the low, “oil canning” , or just plain flexing area. A body line is effectively the “edge” of the panel. Those crowns in the flat panel that I mention end at the body line. So each area in between the lines is sort of like an individual panel. Look to see if the body line is low, it may be holding down your panel. If it is, you need to push it up. To help you determine how straight the line is sometimes you can use a metal ruler as a “straight edge”. How can this be done on a crowned panel you ask? A metal carpenters yard stick will bend very easily, right? So what you do is lay the yard stick on it’s back against the panel and apply a little pressure on the outward edges low area where the metal is OK. You will then have a “curved straight edge”. I have a drawer with a number of these metal or aluminum rulers in it and find them very useful. I treat them like rice paper and they will last a lifetime. So lets say that you have found that you have no low spots in the body lines or there were one or two and you repaired them. Now you have to look for something else that is holding the panel down. This can usually be found in the form of a “crown” or “brow”.
When you put a dent in any panel the metal has to “go” somewhere. All panels have this crown, right? So as an example picture a metal rod that is 3 feet long. This rod has a slight bend to it. The center of the rod is up from the ends about three inches. If you were to push down on the center, the rod would get “longer”, right? So, if the ends of the rod were clamped in vices, the “extra” rod would force the areas on the sides of where you were pushing to go up. You panel does the same thing only on a much smaller scale. Most brows will be found on the outer edges of a panel, this includes of course at the edge of the body line. They are VERY common around the outer edges of a roof. Search around the outer edges of ANY bent roof and you will find them. The brow or crown is a U, C, L or even I shaped high spot. In the center of that curved high spot is a low spot, sort of like a “pocket” in the brow. Just one or two of these will make a panel, especially a large panel look like a cotton sheet! What you have to do is to push up on that low spot while tapping down on the brow.
When I say “tap” I mean TAP. Just the weight of the hammer bouncing off the brow will do it sometimes. Use a large VERY flat body hammer or a flat body spoon for this repair. If you are careful you can repair these brows with little to no plastic filler. Just take you time and keep checking the area with a block with sand paper or a vexon file if you have one for low and high spots.
Now, if you simply can eliminate the brow and low spot, you have won the battle. If it takes some plastic filler, so be it, you have given the panel it’s strength back and that is what matters.