If it ain't broke don't fix it! The following guide was copied from Prusa Printers because there is no better guide out there! All credit goes to them for making this extremely comprehensive guide.
Applying putty and sanding
Now we have the models ready for the next step – hiding gaps between parts and sanding. First, let’s do a rough sanding of the printed object to find out whether we need to add extra material to hide the gaps between different parts. It is also the first step in the process of creating a nice smooth surface. Personally, I prefer the P100-grit sandpaper for this task. Be careful, however, not to make unnecessarily large scratches or gaps in the print, which would require extra fixes afterward. On the other hand, if you just grind through the surface into the infill, don’t throw the part away. You can easily hide it with a putty.
Once the model has been roughly sanded, we can move to fill the gaps between single parts. I recommend using body filler used in the automotive industry. Apply a thin layer on the gaps and let it dry. Once the model has dried, polish it with P100 sandpaper. Keep repeating this procedure until the gaps are not visible. Once the model is ready, we can apply a layer of putty/body filler onto the rest of the model, and then sand it with a P100 sandpaper. And we basically repeat this step over and over (and over) until the model is beautifully smooth.
Filler spraying and grinding
Between each round of application and sanding of body filler, it is a good practice to spray the model with a filler in a spray that will highlight contours between layers, so we can instantly see whether there is some kind of bulge or defect. Use the spray from a distance of approx. 20 cm (8-10 in.) at an angle of about 15 °, this applies to base colors or colorless varnishes as well. Use smooth long strokes and spray the model from one end to the other without staying in one place for too long. This will avoid accumulation of drops of paint. Use a marker to mark areas that have visible inconsistencies and require more care in the next round of sanding/smoothing/painting. You do not have to worry that alcohol-based markers will be visible through the base color and multiple layers of the filler. When the model is finally dry, you will easily mask these marks with the first proper color layer.
Once we have sanded everything to achieve the best possible look, the model needs to be sprayed with a base paint and then finished with a very fine P400-grit sandpaper to achieve a completely smooth surface, which is ready for applying the last layer of base paint. If you feel that the surface is still not smooth enough, you can spray the model with a filler or basecoat again and then sand it with P400 sandpaper – either using dry or wet sanding methods.
Now we have the base paint applied, so we can finally move onto other colors. The best way to apply the paint is to start from the least accessible places or from the largest areas. A good practice is to fix every layer of paint in place using a clear coat spray (glossy or matte) to preserve the layer and prevent any damage to it (such as smudging) during subsequent work.
Then you can proceed with applying other colors either with a brush, airbrush or spray. If you use a brush, keep in mind to mix the color with enough water to avoid visible brush strokes on the surface of the object. Apply multiple layers to achieve proper coverage. The color will be beautifully saturated and the brush marks will not be visible.
If you apply paint using a spray or airbrush, you may need to use a paper adhesive tape to mask already sprayed surfaces. With regard to color selection: brands like Vallejo, Citadel or Model Air will offer you a huge range of shades. Maybe you already have your favorite brands, but I know from my own experience that it’s good to do a bit of experimenting from time to time and discover new products.
Once we have all the colors applied, we can switch to so-called washing, chipping and drybrushing techniques.
Chipping is a technique useful for achieving a damaged surface look. You will need a piece of sponge for this. Soak the foam into the paint, then wipe it into a napkin and start tapping the object lightly with the sponge. (see photo below). This way you can create the effect of weathered/scratched metal for example.
Washing or weathering is a technique for reproducing a weathered look of a surface. It consists of applying heavily diluted paint onto a model with a soft brush. The color is then wiped with a paper napkin in such a way that the residual color creates the “depth” of the model and highlights edges or lines. Applying this color to a larger surface helps to make an object look old or heavily used.
Drybrushing is a technique to highlight the edges of an object. Use a flat brush to pick a color of your choice, then wipe it into a napkin and leave only a small amount of paint on the brush. We use roughly ⅓ of the bristles and move them with a quick movement from one side of the model to the other. The slope should be 90°. This technique ensures that all the edges of the model and details are highlighted.
Finally, fix the last applied color layer of the model with a colorless varnish in spray – shiny or matte, depending on what type of effect you want to achieve. This prevents damage to the color during further manipulation.