Layer lines are the quickest way to make a 3D print look like a 3D print. The good news is that with a little patience, you can finish parts so they look almost injection molded. Sanding, priming, and painting are the three basic steps, and none of them require expensive equipment.
Start With the Right Grit
By Inno Uncle
Begin around 120 grit to knock down the highest ridges, then move through 220, 400, and 600. Wet sanding helps control dust and keeps the plastic from melting under friction. For detailed parts, use sanding sticks or wrap sandpaper around craft foam to follow curves.
Primer Fills the Gaps
A good filler primer does more than add color. It fills small layer lines and pinholes so the surface looks uniform. Spray in thin coats, sand between coats, and repeat until the surface is smooth. Do not rush this part. It is where most finishes succeed or fail.
Paint Like a Model Kit
Acrylic model paints, rattle cans, or automotive spray paint all work. Use light coats to avoid runs. Masking tape and patience will give you cleaner results than any single product. A final clear coat protects the paint from handling.
Of course, the better your starting surface, the less finishing you need. A matte or silk filament can hide layer lines before you even touch sandpaper. And for resin prints, the right resin and curing process reduce post-processing time dramatically.