Using a scratched nonstick baking tray, even if you line it with a baking sheet, may not be entirely safe. Nonstick coatings are made of materials like polytetrafluoroethylene (PTFE), commonly known as Teflon. When scratched or damaged, these coatings can release potentially harmful chemicals into your food.
Even with a baking sheet on top, there's still a risk of the scratches getting worse or bits of the nonstick coating coming off and contaminating your food. The high heat of the oven and the pressure from baking could exacerbate the problem.
To ensure food safety and avoid potential health risks, it's best not to use a scratched nonstick baking tray at all. Instead, consider replacing the tray with a new one, or if you prefer a more environmentally friendly option, switch to stainless steel, cast iron, or ceramic baking sheets and pans. These materials are durable, don't have nonstick coatings that can degrade over time, and are generally safer for cooking at higher temperatures.
If you absolutely need to use the scratched nonstick baking tray temporarily, you can place parchment paper or aluminum foil on top of the tray to create a barrier between your food and the surface of the tray. However, this is only a short-term solution, and it's still best to replace the tray as soon as possible for long-term safety.