Often 3d printed objects may not be in the correct color or may have a rough texture (especially with quicker prints). There are a number of methods for finishing 3d printed objects, included here are simply a few suggestions.
Some 3d printed objects contain hard ridges along the z-axis which may need to be smoothed out for a nice, polished look. Techniques for smoothing out these objects vary, but they basically fall into categories of physical sanding or chemical smoothing.
The most straight-forward method for smoothing objects is to simply sand them with common sandpaper or even steel wool. Any sandpaper will do, although using a sanding pad is often easier on these types of objects. Begin with a coarse-grit sandpaper (80 grit or so) to remove the roughest parts, then work towards a finer-grit (120 to 150 grit) to get a smoother finish.
While sanding is the simplest method of smoothing, the downside is that it can be very difficult to sand complex objects and shapes. It is also very time-consuming, as both ABS and PLA materials are not easy to sand. Often, using a Dremel Tool can help speed up the process, but makes it even more difficult to carefully sand delicate features. Unfortunately, sanding seems to be the only way to smooth PLA material.
Sanding can be greatly improved using other techniques, such as sand-blasting or tumbling. Sand-blasting is the process of using high-pressure air to blow abrasive particles over the object, effectively sanding it down at high speed. Care must be taken when sand-blasting so that parts of the object are not worn away or pits are created in the object. Tumbling is the process of putting the 3d-printed object into a low-speed mixing device with abrasive particles, much like a clothes-dryer or cement-mixer, effectively sanding the object evenly with very low pressure. The constant motion will abrade the surface of the object and result in very smooth surface. Sand-blasting is very fast with the possibility of a less even surface, tumbling is very slow with the likelihood of a nearly perfect surface.
NOTE: This only works for ABS material, PLA does NOT react with acetone.
WARNING: Acetone is a flammable solvent. Prevent any skin exposure, wear safety goggles, and stay far away from any heat/flame sources when using it.
Since acetone is able to dissolve ABS plastic it can be used to smooth ABS printed objects, often providing a very smooth, shiny finish. However you must keep in mind that adding too much acetone to your print may soften it to the point that it collapses or dissolves holes through the object itself.
The easiest method for smoothing a 3d printed object using acetone is to simply pour some acetone into a small cup and use a fine paint brush to apply it to specific parts of your print. By spreading it over the surface, the acetone will dissolve the plastic and cause it to liquify and fill in the imperfections on the surface. Begin by using as little of the acetone as possible and take your time at first, the acetone may still be working even though you don't see any effect immediately so adding more could cause major problems. Until you get a good idea of how it will affect your object, going too fast is likely to result in many destroyed prints. The only way to stop the acetone from working once it is applied is too dilute it in water, although it is likely that by the time you notice a problem it is already too late.
Another method is to convert the acetone into a gas by boiling it in a constrained space, and letting the gas slowly smooth the object in an even fashion. This technique is not currently recommended for obvious safety reasons (heat + acetone = BAD), although we are working on building an apparatus that will allow us to do this safely.
Basic spray paint is easily the simplest means for painting 3d-printed objects. Often the use of primer is helpful, although not always necessary. Most regular brands of spray paint work well, although ironically some paints specifically labelled for "plastics" are among the worst performing paints on plastic objects. I've had very good luck with nearly any version of Rustoleum brands of spray paint.
For more complicated painting projects, standard acrylic paints seem to work as well as anything else I have tried. Acrylic paints are inexpensive, readily available, and dry quickly, plus they can be purchased with a number of finishes including metallic and gloss. A great source for acrylic paints and brushes (for any level of expertise) is National Art and Hobby.
Often the appearance of painted objects can be greatly improved with a finishing coat, which can both protect the paint and provide a more attractive finish to the object. In this situation, spraying a coat of clear finish (polycarbonate, acrylic, satin, gloss, etc.) is the easiest method and works very well. Objects can also be dipped into a finishing material, although you must be careful to avoid air gaps and drips, otherwise it will lead to an uneven coating and even varied thickness of the final object.