Painting 3D Prints (PLA & PETG)

I don't have a ton of experience painting 3D prints, and the few times I tried, I categorically failed. Honestly, I just hate the downtime—having to wait days or hours between priming, painting, and sealing is rough when you just want a finished part.

But I need a reliable workflow, so I'm parking these notes here for the next time I have to tackle this.

The Baseline Workflow

Before getting into the weeds of filament types, you have to understand the core process. Yes, you can technically just blast a raw 3D print with spray paint, but it will look terrible. If you want a clean finish, here is the foolproof order of operations:

Prep & Clean: Degrease and clean the model so paint can actually adhere. A wash with mild soap and water usually does the trick.
Sand: Give the plastic a good rub with 400-grit sandpaper to knock down the glaring layer lines.
Prime: Spray a light, even coat of your chosen primer. Let it dry for at least 30 minutes. This gives the paint something to grip.
Paint: Apply your colors. (Pro-tip: If using acrylics, thin your paint with a tiny drop of water. Two thin coats will always look much smoother than one thick, gloppy coat).
Seal: Finish with a UV-protective clear coat spray to lock in and protect your hard work.

The Prep: Taming PLA vs. PETG

Now that the basic steps are laid out, here is where things usually go wrong. While both are standard filaments, PLA and PETG behave like complete opposites when you subject them to sanding and chemicals (Steps 2 and 3 above). If you treat them the same way, one of them will ruin your paint job.

The Friction & Heat Problem (Sanding)

PLA Melts: PLA has a very low glass transition temperature (around 60°C). When you rub sandpaper against it, the friction generates heat. Instead of sanding away cleanly, PLA quickly softens, melts, and turns into a gooey clump that gums up your sandpaper and scars the surface of your print.
PETG Shreds: PETG has a higher temperature threshold (around 85°C), so it won't melt from hand-sanding. However, PETG is a softer, more stringy polymer. When you sand it, it doesn't turn into a fine powder—it shreds, leaving a "hairy" or fuzzy microscopic texture on the surface that primer struggles to fill.
Action Points:
For PLA: You must wet-sand. Dip your 400-grit sandpaper in water. The water acts as a lubricant and a heatsink, keeping the plastic cool so it turns to powder instead of gum.
For PETG: Sand dry, but keep a heat gun or lighter handy. After sanding, lightly flash the surface with heat to instantly melt away the microscopic "fuzz" before applying primer.

Surface Energy & Paint Adhesion

PLA is Welcoming: PLA has decent surface energy. Once you knock down the layer lines, standard acrylic spray paint and primers will grip the plastic without much of a fight.
PETG is Hostile: PETG is chemically related to the plastic used in water bottles—it is literally engineered to be chemically resistant and non-stick. It has extremely "low surface energy." If you spray standard paint directly onto smooth PETG, it is highly prone to beading up, flaking, or peeling off in sheets later.
Action Points:
For PLA: The standard wash with dish soap and water mentioned earlier is enough prep for your primer.
For PETG: You need aggressive "mechanical keying." Do not skip the sanding step, even if the print looks smooth. You have to rough up the surface so the primer has physical scratches to grip onto. For high-wear items, use an automotive adhesion promoter spray before your actual primer.

The Filler Primer Dependency

PLA: Because PLA bridges incredibly well and prints sharp details, you can often get away with a standard, thin primer coat if your layer heights were low (like 0.12mm).
PETG: PETG is prone to micro-stringing, zits, and slightly bulging layer lines due to its viscous nature.
Action Points:
For PETG: Always use a High-Build Filler Primer (often sold at auto parts stores). This sprays on thick and self-levels, filling in the deeper layer lines and minor stringing pockmarks that PETG is famous for. You spray it, let it dry, sand it smooth, and repeat until the plastic looks like injection-molded resin.

The Golden Rules of Painting Plastic

Once the prep is done, keep these two rules in mind when picking your actual paints:

Rule A: Use "Matte" or "Opaque" Colors for the Base Always look for paints labeled Matte or Opaque. Plastic has a natural glare. If you use glossy paint right over a raw 3D print, the shiny reflections will highlight every single layer line and minor surface flaw. Matte paints absorb light, hiding layer lines and making the surface look flat, injection-molded, and professional.

Rule B: Match Your Primer Color to Your Final Design You need to use a primer. Once primed, the original filament color makes absolutely no difference. However, the color of your primer completely changes how your top coats behave:

Grey Primer (The All-Rounder): If your design uses bright, varied colors (reds, blues, greens, yellows), stick to a neutral grey. It doesn't alter the tint of the paint you put on top of it.
Black Primer (For Shadows & Grime): If you are painting a robotic piece, a dark figurine, or going for a weathered, dingy look, use black primer. It naturally mutes the colors on top. Plus, if your brush misses a tiny crevice, it just looks like a natural shadow instead of raw plastic glaring back at you.
White Primer (For Neon & Ultra-Brights): Yellow, orange, and white paints have terrible hiding power (they’re basically translucent). Putting yellow over a black base requires 10 coats. Over a white base, it pops instantly in 1 or 2 coats. The downside? Any spot you miss with your brush will stand out like a sore thumb.

Extra Tips & Resources

The Magic Keyword: If you want to level up, search for "miniature painting" on YouTube. Understanding just a couple of techniques from the miniatures community before starting a project will do wonders for your confidence.
Spray Brands: Rust-Oleum Fusion is incredible. PLA, PLA+, and PETG all take to it very well without peeling.
Hydro Dipping: If you want to experiment with crazy patterns without hand-painting, check out this awesome Adafruit guide on hydro dipping 3D prints.