Part Finishing

Finishing is where models really start to come together visually. For this important step, it is key that the prototyper pays close attention to all aspects of the surface finish of a model, as the more defects that show up the less likely the prototype is going to be perceived as a product. Smoothing surfaces and adding color by spray painting is a common process for making prototyped parts look like production plastic parts.

Primer is a type of spray paint that helps to prep a part for the color coat. Primer is meant to bond well with a variety of materials, so it acts as a sticky layer in between your part and the top coat of paint. We tend to use filler primer since it is designed to fill in small defects and easily sands to a smooth layer. You will want to choose a primer that has a similar lightness to the top coat color you are choosing, so that a darker primer doesn't show through to your top coat.

Color Coat: The color coat is usually the last coat applied to the prototype, and if you want it to look good the model needs to be properly prepped beforehand. Most of the work of finishing happens in sanding and smoothing the model materials and the primer layer, leaving the color coat as a final application at the end of the finishing process. Even after applying, handle your model with care for the next few days as the paint continues to harden.

Parts off of the PolyJet 3D printers (Objet Alaris or Objet Prime) are mostly solid and are easily sandable. With some wet sanding until smooth, these parts can be directly prepped for priming and painting. Start with coarser sandpaper and work to finer sandpaper: 220 grit / 320 grit / 400 grit.

FDM/FFF parts (Dimension, Up Plus 2, Prusa) are more difficult to prep for finishing because the gaps between layers are much larger and harder to fill. Filler primer alone will not be able to fill the large defects between layers. Start by priming the parts, and then apply a glazing compound or spot putty to fill in the defects. Here we are using AcrylGreen, a fast drying spot putty, and then sanding the spot putty away, leaving only what is needed to fill the gaps behind (180 grit / 220 grit).

Tack cloth, sometimes referred to as cheese cloth, is a sticky, waxy rag that is designed to pick up dust and debris without damaging the finish of your part. Use this type of cloth to clean your part before every spray painting step.

Start off by mixing the spray paint by shaking the can thoroughly. This important step is often overlooked to safe time, but having an uneven spray mess up your paint job will take more time to fix, so it's often best to take the time to prep the can. This is especially important if you have artist spray paint. It's also a good idea to get some gloves on!

When you are ready to spray paint, don't aim the nozzle directly at your part and start spraying, as depressing the nozzle normally let's out a burst of spray paint that may splatter you part. Instead, start painting off to the side, and gently sweep across your part, holding the can 6-8 inches away from your part. Stop and start after each back and forth pass, and rotate your part in 90 degree increments to ensure even coverage from all directions.

Spray paint from back to front. As you add passes of spray paint, any paint that travels to the back of the part will be introduced into the part of the paint job that is still wet, reducing any dusty sections from overspray.

Wait about 10 minutes between coats of primer, and build layers of primer up until you have full, even coverage (usually around 2-3 coats). After priming, wait several hours for the primter to properly dry, stick, and harden. Ideally you would wait overnight, but you'll need at least a few hours if you are tight on time.

After it's fully dry, gently wet sand(320 grit) the primer layer to remove any dust, debris, and light scratches. This preps the part for the final color coat. Spending the time to make the primer layer look as smooth as possible before applying color is the key to a great paint job!

Finally, apply the color coat using the same spray painting techniques for the primer layer. Ideally, one coat will be enough, but you can add another coat after the first coat dries if needed. Once again, careful not to drip! It would be a shame to mess up the paint job after all that hard prep work!

Artist Spray Paint: Montanta Gold is an example of artist spray paint, which is normally sold in art supply stores. These paints come in a wide variety of colors and are primarily designed for being applied to artwork such as graffiti or paintings. While you can use this type of paint on your models, it can be trickier than using hardware spray paint. Here are a few important things to note:

• artist spray paint is at a lower pressure to prevent drips when working with artwork, which means you may need to apply the paint closer and getting full coverage may be difficult
• these paints need to be mixed thoroughly! Shake the cans for several minutes before use, and at least 5 minutes straight if coming from off a store shelf. Use a timer because it's longer than you think!
• be careful with nozzle clogging: since the cans run at a lower pressure, they can be more susceptible to nozzle issues

Hardware Spray Paint: Rustoleum and Krylon are examples of hardware spray paint, which means they are commonly found in hardware stores and used for a variety of home and DIY projects. These types of paints are preferred for creating design prototypes. Some notes:

• since these paints run at a higher pressure, you need to hold the can farther away than an artist spray paint
• extra care needs to be taken to avoid any drips, which are very difficult to remove once present in a paint job