If you do this on a fender, you don't need filler of any kind. The art of metal finishing is not easy. But if you are doing it on these moldings, you are on the way to doing it on your dented fender or door. The most important thing you want to remember is that when you file (a VEXON file for auto body work is the best tool) on something like this, it is more for spotting high and low spots than "cutting" those high spots. You run the file over the area then look closely to see what the file hit. If there are areas where the file hit hard, these are likely high spots that need to be tapped down a little before you proceed. You don't want to keep filing (or sanding) if you are hitting some spots hard and others aren't even being hit. If there are spots that aren't being hit, those need to be brought up. When there are large high and low spots a small hammer or rounded ended “chisel” can be used to “rough” it into shape. All the while, checking with the file, THAT is the tool to see where you are at. I was taught to use a sharp tool like an awl to push up these low spots. It works well because the pressure is so localized. You want to push up from the back with an awl, and DO NOT MOVE where the awl is making contact. As you push up, the pressure is so localized you can usually see on the other side where you are pushing. If you are right on the money, go ahead and bring it up as much as you want. If you are off a little, you can move the awls tip over to where you think you need to be and apply pressure there, again, watching closely on the out side. If you don't move where the awl's point is making contact, you can easily move it a little to where you need to be, remember we are likely talking about moving it an eighth inch or less, sometimes a thirty second of an inch. If you move the awl after pushing an area up, you may not be able to find that exact spot again. The point is (if you'll pardon the pun) to hold pressure on the awl where you pushed up as you look at the outside to see if you hit the right spot. If you do need to remove the awl to file, take a good look at the back and make a mental note of exactly where you pushed. That way you can go right back to the spot if you need to after seeing what the file is telling you. If you don’t move it, you can easily “slide” it over that eighth inch or less to the exact spot you need to bring up. What I do is hold the molding in my hand with my index finger and thumb. In the middle of my hand is the awl. I squeeze the molding pushing the awl up into it. My hand is upside down so the molding is facing up towards me. That way I can easily push up the low spot while I look at it from the other side. The trick is, DO NOT file much, and just run it over the surface until you can see the high and low spots. After you have worked the area to near perfection, you can file a little more to knock down highs to make it perfectly flat for sanding and polishing. But if you rush this identifying of high and low spots and think you can file the damage away, you will end up with too thin of metal and blow the whole thing. When I was learning, I even went thru the metal a time or two. It is easy to do if you ask the file to do too much. The file should be looked at as a straight edge, more than a cutting tool. This process can be long and tedious, you must take your time or you will do more damage than good. If you push up a little too much, just tape it down and try again. We are talking moving the metal a few thousands of an inch or so. Don’t move the metal so much as to make a visible “dent”. Most of the time you will barely be able to see where you moved it (thus the importance of not moving the awl) only the file will tell you if you did or not. Now, once you get the area flat, you have filed it and the file says it is FLAT, you need to sand the file marks out. This is a trick I thank one of my mentors for. I have shown this to many bodymen and they will always be amazed at how good it works. You need a DA, a good old DA that you can lock the head to a grinder mode. Not an “orbital” sander, but a true “Dual action” sander. My favorite is the National Detroit “DAQ”. This tool is the best version on the market in my opinion. It has the most torque using he least amount of air. Ok, you have listen to a story. I mentioned I have shown this trick to many bodymen, well the last time I did, it cost me. I had a nearly brand new DAQ (my last one lasted 25 years!) that I used to demo this procedure on a door skin a co-worker was doing. I was helping him with a problem door and when we came to the point of metal finishing the skin he went to grab his air grinder with 80 grit, NO, NO, NO, wrong tool! I went to my box and grabbed my DAQ with some 120 and quickly knocked the thing down flat and beautiful. There was a few guys there by this time and all were amazed at how well this worked. Well, one of them liked it so much they thought they would “trade” their worn out DAQ for my brand new one. Yep, a few days later I went to use my DAQ and found that in my box was not my new one, but an older abused DAQ!!! To this day I have not found my nice new DAQ, I suspect it is in one of my “buddies” garages at his home. Ok, here is the trick, you use the DA pad as a 6” “block”. If you put the DA on grinder mode with some 600 or 800 on it you can hold it flat as you run it over the surface, favoring the left to right direction. The disc is turning clockwise, so you want to run the disc over the area hitting ONLY the top half of the disc on the metal, against the rotation of the disc. This top area is flat right? It is one half of a perfectly flat circle, so it is in effect a “block”. On this stainless you would use the 600 or finer to “block” it. On sheet metal you could start with 120 or 180, and prime over that. If it was going to be chromed like a bumper that is thick metal, you could still use the 120 or 180 and then move on up to 600 and leave it, the chrome shop will take care of the rest. Heck, I don’t think they use anything finer than that anyway. I have used this method on stainless steel moldings, steel being chromed, thick steel like bumpers being chromed, sheet metal parts like fenders or doors being primed and painted, it WORKS.
The neat thing is, when I was taught this trick back in the seventies, the finest paper you had was 600. Now, with paper all the way up to 3000, you can REALLY do some trick stuff. I had a set of aluminum wheels to sell on eBay. They had some curb rash on the bead. I used this method with 120, 180, 320, 600, 1200, 1500, 2000 and then polished it to a fine luster. It really works well, give it a try.