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What really works well is using a bit of plastic cement or diluted model glue: you apply it to the line and when it dries it creates a smooth surface that you can then sand without risking gouging the plastic like you do with direct sandpaper. Another trick is to try sanding with very light circular motions rather than straight along the seam, so you distribute the effort better and don't dig ugly grooves. In the end anyway a dark wash helps a lot to hide whatever's left, especially on Gundam where panel line shadows are part of the style.
Seam lines can be removed with fine-grit sandpaper (1000 or higher), gently running it along the lines after you've assembled the pieces. If you want cleaner results, use a black or dark gray marker to cover what's left before painting, or apply model putty and sand everything together. Some people also use a hobby knife and file, but it's easier to mess up the part - sandpaper is the safest method for beginners.
If you prefer to avoid sandpaper and the risk of damaging the plastic, try using a pointed tool like a hobby knife or modeling scalpel, running it along the seam with light, controlled movements. It works especially well if the parts are already assembled and the line is clearly visible, though it does take some practice to avoid leaving scratches.
The seam lines never fully disappear, so don't stress too much about perfection. One trick I haven't seen mentioned: gently run a toothpick (or a rounded bamboo stick) along the seam before you use sandpaper - it helps you figure out where the plastic is slightly raised without risking gouging it like you would with a hobby knife.
After sanding, a mistake everyone makes is going straight to primer without cleaning off the plastic dust properly! It sticks to you and ruins the finish, so rinse the piece with warm water and let it dry completely. Another trick that works is putting the model under a strong sidelight while you're working: you'll immediately see where the lines are still visible and you can fix them without overdoing it with sandpaper, which will eat away at everything if you're not careful!
How much time do you have and how much are you willing to invest in tools? 🛠️ Honestly, the combo that's given me the best results is starting with a modeling scalpel (really light movements, no rushing) and then finishing with 1000-1500 grit sandpaper, which is the real game changer for smoothing out what's left. Sandpaper alone risks rounding off the details, but if you use the scalpel first to take off most of the line, then the sandpaper finishes the job clean without messing anything up.
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