Can You Paint PETG (How To)? Should You Sand It!


PETG is a derivative of PET, the plastic used to make everything from your coke bottle to advertising displays. 

Unlike PET, however, PETG does not suffer from weaknesses like overheating or fragility. Its improved strength and odorless printing are partly why it is a favored alternative to more popular filaments like PLA and ABS. 

Post-processing your PETG print can help improve how it looks as well as its resistance to heat and moisture. 

Painting a PETG print is a commonly used way to make it more weather-resistant. Adding a splash of paint has the double advantage of making it look better and protecting it from the elements, especially if it will be used outdoors.

But how do you paint PETG and what type of paints would work on it? 

Can You Sand And Paint PETG?

The layer-by-layer deposition of 3D FDM printing leaves visible layer lines on a model. Sanding is one of several techniques used to smooth PETG prints. This improves the surface appearance by removing the layer lines left by 3D printing. 

Painting is a post-process that not only improves the finish but also forms a substrate that protects the print underneath from heat, chemicals, or moisture.  

Post-processing is not always necessary, for example with a functional item like a gear that forms part of an engine. In this case, sanding and painting might just sink unnecessary time and effort for something that has a functional and not aesthetic purpose. 

Perhaps the only reason you might consider painting here would be to protect it from moisture. Sanding as we’ve mentioned removes the layer lines in a print. Sanding PETG, however, can be time-consuming. Alternative smoothing methods that do not take as long include

  1. Coating using an epoxy resin.
  2. Heat treatment using a heat gun.
  3. Dissolving in a chemical bath. Source
  4. What Kind Of Paint Do You Use On PETG?

In theory, almost any type of paint can be used on plastic. In practice, certain finishes like water-based paints for example will not produce the best results. Water-based paints like PVA for example wash off with water. Aerosols and acrylics are the easiest to apply and absorb into a plastic well.

Prepping PETG Prints For Painting (2 Steps)

Before painting your prints there are two preparatory steps that you should take to make the paint bond tighter to your print.

Sand your prints

Painting a print with an uneven surface is a sure way to ruin the finish. The paint will not spread and absorb evenly. The gaps and cracks are the first places where the paint will start to peel if you don’t sand them. Start with a hard-grit sandpaper and work your way down to a finer grit to remove the rough edges of your print.

Prime your prints

Priming provides a better surface for your paint to stick to compared to applying paint directly. Use an appropriate primer for the specific type of paint you use.

Painting PETG With Aerosol

Aerosols (spray cans) can produce a more even finish than using a paintbrush. They are easier to use since you don’t have to know how to paint with a brush. 

  1. Shake the can to mix the paint before you start.
  2. Paint from the edges of your print working your way in.
  3. Go over your print in slow strokes, making sure not to pause over an area as this may create clumps.
  4. Paint one coat at a time allowing time to dry between coats.

Will Acrylic Paint Stick To PETG?

Along with aerosols, acrylic paint can be used to paint PETG prints. As with aerosols, the adhesion of acrylic can be improved by first priming the print. Paint can then be applied to the primed surface using a paintbrush.

Prepping for acrylic paint follows the same step as with an aerosol. First, you need to smooth your print to remove uneven edges that can create weak points where paint is more likely to peel off. Then apply the primer and allow it to dry. 

  1. Get a quality paintbrush. 
  2. Apply between two or more coats allowing time to dry between coats.
  3. Paint in a clean environment free of dust that might stick to the print.

Does Acetone Melt PETG?

Acetone is a solvent commonly used for dissolving ABS. While it completely dissolves ABS, it has very little to no effect on PETG and will not dissolve it. Tetrahydrofuran (THF), ethyl acetate, methyl ethyl ketone (MEK), and dichloromethane are more effective at dissolving PETG than acetone.   

It’s important to note that these are highly toxic chemicals and proper safety should be observed when working with them. 

  1. Work in a well-ventilated room.
  2. Wear a dust mask and gloves to prevent inhalation or contact with the skin.

While it might not work with all brands of PETG nor be as effective as other solvents, ethyl acetate is less toxic than the other options. If you have to use a chemical solvent, start with it. You should also consider using alternative methods like a heat gun. 

Does PETG Scratch Easily?

PETG can be used to create transparent 3D prints. It can be used to make displays, covers, and anything that requires a clear finish. Acrylic is also used to make the same type of items because it is a transparent filament. 

Between the two, acrylic might be the better option for transparent applications, however. The main advantage of using acrylic is that PETG scratches more easily compared to acrylic. 

Polymethylmethacrylate (PMMA) or acrylic filament is great for producing transparent or semi-transparent prints. Acrylic filament is generally not used as much as it is prone to warping and is at least five times weaker than PETG. 

With that being said, acrylics are generally cheaper, produce a clearer item, and are more scratch resistant than PETG. Source

Will PETG Warp In The Sun?

PETG has a glass transition of between 80 to 85°C. The heat from the sun will not warp a PETG print. PETG has a high resistance to the UV-A and UV-B radiation of the sun. This is one of the features that make PETG prints more suitable for outdoor use than PLA.

Glass transition is a measure of the temperature at which a plastic goes soft when exposed to heat. Different plastics have varying glass transitions. PLA for example has a glass transition of between 60 to 65°C while ABS has a higher glass transition, going soft at 105°C. Source

sherifjallad

I am a very well experienced techie civil engineer who's extensively interested in 3D printing technology and even more captivated by the potential of 3D printing livable structures

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