Painting is 90% prep, 10% spray. I painted my '26 myself in a relatively clean garage and it came out extremely well. Roughly 130 hours of body work(original 83 year old steel body so you can imagine how rough it was) and 5 hours of actual spraying.
You can take this in a couple directions. You can sand off all the original paint to bare metal, which is what I did, or you can scuff the factory paint. Imo bare metal is the way to go if you ever want to paint this car again or foresee yourself painting it again.
However if I was in your positions with I assume, little to no body work experience then I would just go ahead and scuff the factory paint. Most factory paint is baked on at 450 degrees and if you simply try to paint over this you will have problems with flaking and discoloration down the road. In your post you said that you already have a problem with that stuff so you may need to sand to the paint on the affected panels. The best route to scuff paint is to use a fine-grade scotch-brite pad. 500-600 grit works wonders on factory paint. You do not need to sand through the clear if the panel is in excellent condition(Not flaking, discolored etc). It's a waste of time and energy. All you need to do is get a nice surface for the new primer to grab onto.
Couple tips for scuffing is that you do NOT need to go in a single uniform direction. In fact I would advise you to go in many different directions to get the best possible results out of your primer. Also if you have an problem areas that need filled in with body filler such as dents etc. I recommend sanding that surrounding area down to bare metal unless the body filler you are using specifically says it can be used over paint. I use Rage Extreme filler, which can be used over scuffed painted surfaces. Downside to this is that it is extremely expensive compared to Bondo brand filler.
Not sure if you have very many dents or uneven surfaces but if you do here are some tips for Body Filler. I personally prefer to go the extra mile and do metal work rather then plastering body filler over the problem area but then again I have all the tools to do it and I was taught in person by someone with decades of experience, all the little tricks to it. Plus I am pretty anal about perfection and even though nobody would ever be able to tell the difference between body filler and actual metal given proper coverage I would still know it's there.
First thing you need to do is get a nice, smooth pattern going with the filler. Bondo brand filler is a little harder to work with the Rage Extreme because it's a thicker base. Either filler works but the smoother you can spread the filler over the problem area the easier it will make the rest of the process.
Hopefully you can spread your filler on smoother then that. You want to avoid the lumps and valleys that this guy has going on. Also notice how he just plastered the filler right over the accents in the fender. You want to avoid this at all costs. It is an absolute PITA to get filler out of areas like that without screwing up the rest of your body work in the surrounding areas.
At the end of the day you are going to spend 5 times as long sanding that down to get that nice, smooth, and most importantly FLAT. If your body work is not flat you will see ripples in the finished product.
After you have gotten your filler on you need to sand it. You can use an orbital or a jitterbug sander. I personally would not. Use 80 grit dry sandpaper to start. If you don't have a sanding block(below), jitter-bug, or orbital sander you can use a method called cross-sanding. It's basically as simple as it sounds. You want to triple fold your sandpaper. So that your fingers will be resting atop 3 layers of paper. You want to sand in a vertical, horizontal and diagonal pattern.
The purpose for this is to reduce your chance of getting finger marks/valleys in your work. You also want to use this technique in areas that you can't get your sander/block into. Such as round corners around windows. License plate frame area, around your spoiler, and other such tough to get to areas like that.
After your initial sanding of your bondo you may have pin holes in them such as these below. Sorry about the horrible picture. It's the only one I could find.
What you want to do is use a second layer of filler over the area and then sand over it with 180 grit dry paper. With the cheaper Bondo brand filler this may open up more pin holes with the higher grade fillers it probably won't. If it does just use a 3rd layer and that should clear up any pin holes that you have.
When you are finished you should have an area, variable in size, that looks somewhat like this.
This guy has old filler, primer and paint still on the piece. You can see the body filler in white/grey. Notice the irregular border of the filler and bare metal. This is a sign of flatness. If you have a straight border on your final sanded filler then that most of either been one hell of a straight dent or the filler is not contoured to the piece you are working on.
To tell if your piece is "flat" you want to run your hand over the surface. Stiffen your four fingers and bend them so that your palm doesn't touch the surface you're gauging. You want to feel it in a horizontal, vertical, and diagonal pattern. You WILL feel lumps if it is not perfectly smooth.
It is up to you how smooth you want to go. It's certainly not easy(in regards to time) to get a perfectly flat surface and it will take some effort to get the surface flat.
Just remember that the work you put into the body WILL be evident in the final product. You can not cut corners and expect a perfect finish. I can give you more tips and pointers in regards to the actual priming, painting, clearing, and the wet sanding process if you would like me to.
Edit
Pictures of my '26 so you know I'm not just blowing smoke. ^.^
Prior to clearcoat sanding and buffing.
After buffing.
