Yes, you can paint polyethylene, but it requires specific preparation due to its low surface energy which makes it non-reactive and challenging for paint to adhere to. Start by thoroughly cleaning the surface with soapy water and then rinse it with clean water. After drying, lightly sand the polyethylene surface to create a texture for the paint to grip. Wipe off the dust, then apply a primer designed for plastic surfaces. Once the primer is dry, you can paint it with a paint suitable for plastic. It’s important to use spray paints or paints specifically formulated for plastic surfaces to ensure the best adherence and durability.

Thickener 415, also known as Xanthan Gum, is indeed gluten-free. Derived from the fermentation of glucose, sucrose, or lactose by the Xanthomonas campestris bacterium, it's a popular thickening and stabilizing agent used in various food and cosmetic products. Despite its bacterial origin, it does not contain gluten, making it a safe option for individuals with celiac disease or gluten sensitivities. However, it's always prudent to check product labels, as cross-contamination during manufacturing could occur with products containing gluten.