Getting rid of seams

The object of modeling is to turn a collection of parts into one sculpted piece. For a very young modeler, just getting the thing glued together is challenging enough, so I wouldn't worry about doing anything more than putting on a simple paint job. For you older folks, a little more effort will give you even better results. This means getting rid of the seams and joints where you glue the various parts of the model together.  Dealing with this is really not hard or complicated, it just takes some careful work and a few basic supplies. You have both the mold lines left on the pieces from the injection molding process as well as the gaps and lines where you join the pieces together. Mold lines are fine ridges that run across the part, and usually just take some scraping and sanding.  The joints where pieces go together need more work and usually involve the putty.

Start by taking the exacto knife and scraping along the line. This will take off any high spots and glue residue, and sometimes this is all that is needed. Then use the files to get the worse areas shaped, for instance where a seam doesn't match up at all.  Finally go to the sandpaper (wet), starting with the 400 and finishing with at least the 600.  You should now have a seam that looks and feels level. By the way, you'll be using a lot of sandpaper, so instead of buying the expensive little packs in the hobby rack, I highly suggest you wander over to the automobile section where they sell bodywork and painting supplies. You can buy big sheets of wet-dry sandpaper in assorted grades for half the price.

Now, after this initial work with the sandpaper, take a good look at your model under a bright light. You can usually see obvious gaps and low spots at this point. This is what the putty is for. Different modelers swear by different brands, and the ones I use are Squadron Green and Testors Contour Putty, both readily available in hobby stores.  Ask around on the net, and you'll find people who use other products like two part epoxy or gap filling superglue - this is one area where it seems everyone has their favorite and swears by it.  To apply, I put a dab of putty on my finger and wipe it along the seam, making sure I get it pushed into the cracks. I might then smooth it a little by then wetting my finger and going back over it. Another trick I use is mixing a little acetone (fingernail polish remover) and putty in a small pill bottle, and using a brush to dab it on where needed.  Use thin coats, it's fast drying but still one of the delays in building is waiting for the putty to dry completely.  I usually give it a whole day before hitting it with the sandpaper.

Once the putty is dry, use the sandpaper and files to get a smooth surface that still matches any details on the model, yet hides the seam or defect.  It's better to take your time with this step and remove too little putty and plastic at one time than too much.  After each session with the sandpaper, put on a coat of primer over the area you're working on.  You can buy special stuff, I just use a neutral gray acrylic.  You will usually find this allows you to see smaller areas needing more sanding and filling. Repeat this process until a coat of primer shows no evidence of the mold line, and you're finished.

A "diddle" stick comes in real handy when sanding. This is a plastic or wooden stick about the size and shape of a regular screwdriver. You can make your own from something like a popsicle stick or find something even better by scrounging around. Wrap the sandpaper around the end and use it to get into corners and shape details.