What you need to paint door frames: first: you clean with spirit and water, like cleaning windows, just without soap! That also degreases at the same time. Acetone also works but is more toxic. Second: you sand with a sanding block and wet sandpaper, first 200 grit, then 300 grit (250-350 grit also works, the main thing is first coarser, then finer). Wet sandpaper because then no dust is created. It is indoors, after all. The wet sandpaper is really dipped in water, the frame moistened with a sponge.. That’s how it is. After sanding, there should be a smooth, matte surface. You dust it off. Swiffer cloths work well. Then a break. The work wants to mature in peace. Then tape off with Tesakrepp or a similar product. On the floor, on the edges. When painting, you shouldn’t think about avoiding overpainting, but paint quickly without hesitation. That’s why tape off! Now the paint. Buy good paint. Get advice. Good companies would be: Herbol, Sto, Sikkens/Akzo Nobel for example. Use alkyd paint or PU paint. It contains solvents but levels better than water-based acrylic paints. The surfaces become smoother for amateurs. Buy paint rollers for solvent-based paints. Ask specifically for that! They are fine-pored and create a smooth surface, the orange peel texture disappears with good paint. For rolling, pour some paint into an empty, clean small potato salad container or similar. Roll the frame once—not too thick, otherwise there will be drips and runs. Let it dry. Next day, second coat. That should be enough. The whole procedure only requires patience. Oh yes. Is there a rubber seal inside? That obviously goes out before painting. And remove the door leaf. Have fun. It is an almost artistic, meditative activity.