Steve, I was first introduced (like many modelers my age) to "Wet-or-Dry" sandpaper WAY back in the early 60's when AMT Corporation put out their line of "Styline" kits, model cars with a tube of putty and a small sheet of 400-grit wet-or-dry sandpaper (the black stuff used to this very day in autobody shops everywhere. I always keep a pack of 3M 400-grit sandpaper in stock in my model room to this very day. I simply cut it into smaller sheets, which I then fold with a sharp crease, and use wet on a model car body, to gently remove mold parting lines (those often raised, sharp lines where the different mold sections have to come together on a model car body, in order to mold it as a one-piece unit. (Bear in mind, that in order to make a one-;piece plastic body in styrene, it takes a minimum of SIX mold sections, which "slide together" as the mold closes up, to make that body. It's much like looking at a cube, or a rectangular box: a mold section to make the upper surfaces, a right side mold, a left side mold, a front end mold a rear end mold, and the sixth one to make the inside of the body shell. No matter the manufacturer, this practice does not change--only the precision (or seeming lack thereof will change). Now, with most all modern-made model kits the parting lines (where those mold sections must meet and close up tightly) will be there, and they will show, of course, through a paint job unless removed. This means working often in tight places, which is why I use small bits of 400 grit to get at those. It also means working carefully, sometimes quite delicately, so that I don't damage or remove the raised detailing that I paid good money to get in the first place--just the offending, but necessarily present, mold lines.
Sometimes, particularly with older kits, those tooled in years past, before truly modern toolmaking reached the model kit industry as it is today, there may be places where the tool sections do not meet up exactly precisely, meaning there can be some misalignment (if this is the case, that's were a set of really decent "needle files" can come in handy--but again, planning, and learning to work very delicately has to come in!), and there may be a bit of putty work needed sometimes as well. Again,think about how to work putty in small areas (I use the very smallest artist's painting knife I can find (my current painting knife, in stainless steel, as a blade shaped much like a miniature masonry trowel, is polished stainless steel, and was milled to a tapered thickness, which gives it a good bit of "springiness" which I find both advantageous and critical! Its nose or front tip is rounded as well. Where to find one? Look no farther than the art supply section of Hobby Lobby or Michael's, or any good local art supply store--those painting knives will be right there in with the artist's oil paints or acrylic artist's colors (the stuff in those little "toothpaste" like tubes--I paid less than $10 for the one I use, but that was about 10 years ago now--might be a bit higher these days. For putty, I prefer autobody spot n glaze putty, the one-part lacquer-based putty, found in the autobody supply sections in most Big Box stores, or if you have a professional level auttobody supply store (I have one about 6 blocks from my home), and some lacquer thinner (I use Kleen Strip from Walmart, and a small jar (a glass baby food jar works great!), for cleaning up my painter's knife after each use). Let that putty dry hard, then sand smooth.
Long-winded, yes I know, but this is stuff I've learned how to use and how to do, over now more than 50 years of building model cars.