Hi Ethan, I’ve had similar challenges in the past. The key is making the surface chemically AND mechanically receptive to the paint. A roughened surface with some broken polymer chains will help solve your challenge. Here are a few steps you can take to improve paint adhesion:

1. Start by thoroughly cleaning the plastic surface to remove any contaminants, dust, or grease. Use a water solution and mild detergent, followed by wiping the surface with isopropyl alcohol.
2. Lightly sand the surface with fine-grit sandpaper (around 400-600 grit). This helps create a key the paint can adhere to better.
3. Applying a primer specifically designed for plastics can significantly improve adhesion if you find the right one. Brands like Rust-Oleum and Krylon offer plastic primers that can work quite well. Apply multiple, thin coats, this can help keying.
4. Use a paint that is suitable for the surface. Acrylic paints and spray paints designed for plastics generally offer better adhesion and durability, having tuned solvents in them to suit various plastics.

Hey Ethan, in addition to plastic primers, consider using an adhesion promoter. These products are designed to chemically open the surface to help with bonding. They’re really needed for difficult-to-paint plastics like polypropylene and polyethylene but can help with easier materials too. 

Flame treatment can improve paint adhesion. Carefully passing the plastic part over a propane torch can depolymerize the surface just a little, allowing some improved coupling – but don’t overdo it! Plasma discharge can achieve the same results but with less heat.

Pre-heating the plastic part before painting can sometimes help. Warming the plastic (not too hot, just warm to the touch) can kind of relax the surface, improving the coupling. Actually, it’s sometimes good to use several of these methods, so you’ll need to experiment a bit.

Yes, certain types of plastics are more challenging to paint due to their really low-energy surfaces. As a rule, if it’s slippery or greasy-feeling, it’s going to be hard to paint. Teflon, polyethylene, and polyprop are all hard to work with.

For these plastics, using an adhesion promoter (specialized primer), plus flame treatment and surface roughing will often yield results when you do all of them together! You’ll also need to use paints specifically formulated for these difficult plastics.

Just to add to what Mason said, for the most challenging plastics like PP and PE, consider using two-part epoxy paints, as these can sometimes offer better adhesion and durability on tough plastics. They can provide a stronger bond.