Even new cars have depressions in the panels around door handle stampings and places like that. Watch those old 1930s assembly line videos. the speed they built those cars, and you'll know that these cars did have perfect panels.
If you can feel a depression with the flat palm of your hand it's too deep to block out with primer, especially thesed days with the price of paint. If it's only 1/32nd deep I'd just skim coat it with body filler, it's not going to crack and fallout if the metal is prepared properly. Applied over clean bare metal with 36 grit scratches so it has something to hold onto.
When you're sanding the filler, you will start to see the metal break through around the edges of the depression. At that point you are really close to being perfect. If you keep sanding until there is definitive line between filler and the metal, you have gone too far and created a low spot. You will think you will be close enough to block it out with primer, but it would take several heavy coats to do it. You can save a lot of time and money by learning when to stop sanding on the filler. The body Filler should be sanded with 36 grit speed file paper just until the metal starts to show around the depression. Almost to where the filler is so thin you can see the metal through it. There will be filler on both sides of where the metal is starting to show. Then hit it just with some 80 grit to knock off the large 36 grit scratches. Then sand off the filler around outside of the depression but stay away from the filler over the depression, let the filler feather out into the metal around the low spot, if you keep sanding, trying to move all the scratches from the filler you will end up with a low spot every time. Don't push down hard when your sanding, it flexis the metal panel and you wont get a perfectly flat blocking job. Let the sand paper cut the filler without forcing it. That is the way I was taught at a college autobody class 40 years ago by a friend of my dad that was a master metal finisher.
I've tried metal finishing and unless you're really good at metal finishing and practice daily you will probably remove too much metal. I don't do it enough to be good at it. I prefer to hammer and dolly, shrink... do whatever I can without grinding and filing any metal away. I get it a close as I can and then I skim coat it with filler.
I prime over the filler and water sand it with 400 grit. Then I take a paper towel and wipe the panel with wax and grease remover. That makes it momentarily shiny and you can look down the panel for any waves in the paint. If there is the slightest wave in the paint you will see it with the wax and grease remover. There are no shortcuts in automotive body and paint work. Its tedious work that requires lots of patience. You can't say, I'm going to work on a panel this weekend and then paint it. It will be ready paint when that guide coat of wax and grease remover says it ready, weeks, months... Its hard to finish a perfect paint job without burning out on the project.
Here some pictures.
50-years-ago, dad and I water sanding 1929 Woody wire wheels. He did his own paint work with a small 15 gallon Sears compressor and a clothesline for a paint booth. We would go to the Ditzler paint store, and he would walk out with a gallon of black lacquer, a gallon of grey lacquer primer, a gallon of red lacquer primer, a gallon of fast drying lacquer thinner, a gallon of slow drying thinner, a gallon of the cheap gun cleaning lacquer thinner. lacquer spot putty, sand paper and he would ***** about the $30.00-$40.00 that it all cost. Then we would go over to Standard Brands, you brought your own gallon can and they filled it with solvent for .99 cents
He bought a '40 Ford in the 1950s and he and the seller became friends lifelong friends. That guy was the one that taught me body and paint at the local college when I was 18. I can do a decent job but I'm so slow at it.
My dad had a lot of '40 Fords. This is my mom with one of them. A friend was learning how to paint flames and did these silly flames for free so he could practice. At the time my dad was a civilian working on the air force base painting airplanes. For years growing up its was Model A's around our house and then it was early Mustangs and then if was flatheads. The fake Shelby was my high school car. The gold convertible I bought when I was 18. The white fastback was my dad's daily driver the orange '64 1/2 convertible was my little sister's. The funny thing was the family next door also had three early Mustangs. I was brained wash into being Ford guy. I never got a toy car that wasn't Ford. My first pedal car in 1965 was a Mustang, Every hot wheels was a Ford.
