…with the aim of looking in-character on the cheap.
So you've bought a dart gun (I recommend second-hand off eBay for cheap bundles of them), but unfortunately it's a nasty lump of coloured plastic. Time to paint it so it looks more IC! Compared to standard LRP weapons, a dart gun is easy to find and fairly cheap, but needs a bit of work to feel good threatening someone with it in a tense, high-immersion combat situation.
Thankfully, painting up a dart gun to look good is fairly straightforward even for the artistically disinclined.
If you don’t want to take your gun apart, then don't. You can get a pretty good paint job done without cracking it open, particularly if you're doing all your painting by hand rather than spray paint.
That said, most dart guns are pretty easy to take apart and reassemble, although you need to make sure you keep all the screws and springs, and possibly consult some youtube videos or record your deconstruction with photos in case you forget which internal components go where. Most Nerf guns use relatively small Phillips screws throughout, but sometimes they can be set pretty deep in the casing, so ideally you want to use a precision screwdriver.
The gun in this guide is a Nerf Doomlands Persuader, which has a transparent panel that I wanted to keep that way, so opening it up and painting the inside was pretty important.
Whether you have cracked open your gun or not, you will probably want to get rid of the raised logos, toy safety marks, etc. - these will show up even more in relief when you have painted the gun, so you definitely want to get rid of them. Use your coarse sandpaper to sand them down until the non-raised surfaces around the test starts to show scratching. You can tear or fold the paper into smaller bits to get into cracks or avoid sanding down parts you want to keep. You should wear a dust mask while sanding.
Next step is to provide a key in the smooth plastic for the paint to adhere to. Use your finer sandpaper to roughen the surface of any exposed parts that you intend to paint. Avoid going into the dart-holding components or internal mechanisms like plungers - you dont want to make jams more likely by gunking up these mechanisms too much. Once you've sanded everything, rub the whole thing with a damp rag to get the dust off.
You'll need a good base coat to prevent the plastic of the gun showing through and building up a good surface for your colour layer. Grey is a pretty all-purpose base coat colour, but use black if you want a darker colour scheme (and for metallic parts for reasons explained below) and white for brighter colours (although it will take a lot of coats to build up a good white base). You might need to do a lot of coats, both at this stage and at later stages - if you are painting by hand you can speed up the time between coats by using a hot hair dryer to dry everything out.
A really good technique for parts you want to have a metallic look is to drybrush a black base coat with your metallic paint. Drybrushing is a bit messy, so do it before you do your main colours. Dip your brush in a bit of paint, and then brush off most of the paint into the pot or on a bit of newspaper, so the brush has a layer of pigment on it, but there isn't really any liquid paint left on it. Then use light strokes going back and forth across the area you want to colour - this is a nice and quick way to give a bit of colour on a dark background, and looks great for metallic areas (although works for other colours as well).
Once you've base coated, build up repeated coats of your main colour(s). Use lots of coats rather than just slathering the area with paint - it takes longer, but the colour will be more consistent, with fewer brush marks and better control.
Note on mechanisms - some dart guns have mechanisms built in that might become less smooth to operate with a few layers of paint on them - flick-out six-shooter barrels and the like. You may have to balance aesthetics with functionality when you decide to paint these.
If you're a veteran painter, you can go on to add detail, highlights, burn marks and other battle damage to your gun, but we won't cover that here - there are innumerable tutorials on the internet and youtube to get those sorts of looks.
You could look into various effect paints to add detail and give a unique look - iridescent paint will add sparkle, phosphorescent (proper glow-in-the-dark) and flourescent (glows in UV light) paint could give you nice effects. If you are an amateur electronics enthusiast, you could wire up some LEDs to a battery, and have the whole thing light up when you pull the trigger or something.
Once you're done painting, you might be happy enough to stop there. The paint should be fairly resilient, and if you've done enough coats will be pretty unlikely to chip. However, a few layers of varnish will give you a smoother feel, and using a gloss varnish will add a nice sheen if you want that.