Absolutely hate mold lines.
Q&D way to do it is to slap Vallejo Plastic Putty on the seam, then scrape and trim with a hobby knife, and file down with the jeweler's files. Engraving pens help as well. How good are sanding sticks?
I also paint boardgame plastic (yuck!) and scraping off soft plastic is a pain! Serves me right for being cheap!