Unpaintable plastic

Due to Prowl's mold layout, changing the outer shell of the car while leaving the limbs white for Sideswipe left a white part on the roof that had to be covered by a sticker. All of Sideswipe's white plastic (and the corresponding parts on other versions of the mold) cannot hold paint.

Just what its name implies, unpaintable plastic is plastic to which the type of paint Hasbro and TakaraTomy use will not properly adhere, due to the chemical compositions of said plastic. This plastic is sometimes used on Transformers toys for structural reasons, due to its durability, flexibility, or resilience. These qualities are center to the materials' usefulness, but also the source of their unpaintability: their high resistance to chemicals, particularly the solvents used in paint application.

The two most common plastics that are unpaintable and often used in Transformers are Polyoxymethylene (POM) and especially Polyamide (PA). The vast majority of Transformers toys have at least one mold that is unpaintable.

Examples

• Beast Wars Dinobot's beast forelegs, the piece at the back of his beast mode neck, his robot shoulders, and his robot thighs are unpaintable plastic.
• Armada Thundercracker's silver plastic is unpaintable, which is why his eyes are silver and indirectly why his face is painted red. (Thundercracker's face is usually silver, but since his eyes and face would have been the same color, his deco artist changed Thundercracker's face color.)
• The Ultimate Battle Optimus Prime — Much of his red plastic, especially the upper arms, is made of unpaintable plastic. This is why the Autobot symbol tampograph was moved to his forearm.
• OTFCC 2003 Sideswipe & Sunstreaker — See photo and caption to right.
• Prime: Robots in Disguise Weaponizer Class Bumblebee has unpaintable plastic between his front wheels and doors, which is usually where his stripes would pass through. This is partly why his vehicle mode deco was entirely changed for the final product.
• Robots in Disguise Legion Class Strongarm's thighs. A paint operation planned for her kneecaps was removed due to this.
• Combiner Wars Dead End and his mold derivatives have unpaintable elbow joints. For Hasbro's versions of Dead End, Brake-Neck, Prowl, and Smokescreen, the elbow piece is molded in a color matching the car mode's side deco color, while the Unite Warriors versions of Dead End, Wildrider, and Prowl use plastic that matches the primary car mode color, as do both versions of Protectobot Streetwise and the Torchbearer Dust Up.

Photodegradation

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While it is nothing new, photodegradation has become much more common and rapid in products released ever since the War for Cybertron: Siege toyline. Several figures' unpaintable plastic has become increasingly susceptible to decay and discoloration even while still inside the packaging or mere days after being opened. These figures are also known to "yellow" even when they aren't exposed to direct sunlight—even when inside windowless boxes!—and have been observed to have an accelerated reaction in humid climates.

The precise cause is still unclear. A common theory among the fandom is that a fire retardant within the nylon chemical buildup is the cause of this and reacts to heat and/or humidity, causing the plastic to yellow. There was even one case in TFWiki.net's own community Discord server where—what is believed to be—the yellowy-colored fire retardant chemical seeping out of a seam in an individual's War for Cybertron Trilogy Soundwave's knee! Gross. This was most notable as their figure never suffered the same fate of the rapid yellowing that just about everyone else had endured, likely due to the cause seeping out of the plastic instead. Other proposed explanations blame the [[wikipedia:{{#if:|:}}COVID-19|{{#if:||COVID-19}}]] pandemic: rapid changes in factory staff and cleaning methods, reduced feasibility of Hasbro site-checks, longer time spent on slowed-down Pacific cargo ships, or some mix of the above. As noted above, the newer sudden-onset phenomenon was first noticed around 2018, two years prior to the world-wide outbreak, so some of that may not be the real cause - though it could certainly have gotten in the way of fixing it.

Notes

• Before you ask; yes, you can probably get commercially available hobbyist paints to stick to this plastic. However, you are not a multi-million dollar toy corporation that needs to meet stringent testing and durability standards while staying within very stringent budget constraints. This fundamental disconnect is pretty much the entire driving force behind customizing.
• That said, if you're going to try and paint that kind of plastic, you need to be keenly aware of the limitations of your options. Enamels are a bad idea, as they chemically bond with plastics, and the results can end up gummy and sticky forever when applied to softer plastics. Acrylics form a "shell" over the plastic with minimal bonding, which is preferable. Still, those can chip and flake off fairly easily. A clear-coat layer can mitigate this, but since most unpaintable plastic parts are joints where a lot of friction happens and there's minimal room for more layers of anything, well... good luck.
• Speaking of painting unpaintable plastics, it has not stopped Hasbro Asia from either fully plating/coating or weathering/detailing them for their Lucky Draw and their "Battle Damage" figures.