Anything reflective should work but it has to reflect short wavelength. Anything polymer could have a transparent layer on top which could (partially) filters the UV an other short wavelength and heats up the dish instead.
You can also remove the paint and mirror polish the dish itself but that requires tools and it likely will not stay shiny for long.
But UV is what destroys the polymer if it does not reflect it.
Beside that if something is "glossy" and reflects UV like tinfoil (Al) it will almost certainly reflect most of the visible light waves and part of the IR too and that is what you want.
In contrast anything transparent like a clear coating on a polished dish, is very likely only partially transparent to visible light.
It turns out after some reading that a reflective BO-PET should be reflective enough initially but will yellow and become brittle within fewer than 1000 hours of sun exposure. So as a very short-term use in an emergency if it's what's available to build a solar still, oven, or forge it could do the trick if it's the only thing available. Some metal foil should be far more durable though.
The boPET layer in question is thin enough that it will have to yellow enormously before that becomes a problem. On the other hand, metal foils, other than gold and some of the platinum group, are not that good at resisting chemical weathering in the air, especially when water and sulfur are present. So it wouldn't be surprising if, in the rain, boPET-covered aluminum remained a usable mirror without repolishing for longer than unprotected aluminum. Also, replacing aluminized boPET might be both easier and cheaper than repolishing solid aluminum; the amount of aluminum required per square meter is about two orders of magnitude smaller than with aluminum foil. This holds a fortiori for silver.
To correct an earlier poster, tin is not aluminum. Although tin was responsible for the high reflectivity and corrosion resistance of Newton's mirrors, aluminum is far superior on both counts.
Gold has higher reflectivity than aluminum in the infrared but lower in the blue. Because so much sunlight is NIR and so little is blue, this turns out to be an improvement for preventing things in LEO from heating up, which is why astronaut helmets use gold rather than aluminum or silver. I don't know offhand if the same is true of the solar spectrum at the bottom of Earth's atmosphere.
Tin foil and aluminum foil are two different things, although many North Americans do not know this. Tin foil (once commonplace, now hard to find) is made of tin, while aluminum foil is made out of aluminum, which is a different metal from tin. For many purposes the differences are not very important — both are low-melting-point, white, corrosion-resistant, food-safe metals that can take a high polish — but for solar-collector mirrors the differences in reflectivity and corrosion-resistance matter a lot.
This conversation reminds me of a guy on Reddit who learned that old-timey whitewash was lime, so he tried painting things with lime, but it was gardening lime (calcium carbonate) rather than slaked lime (calcium hydroxide), which is what holds whitewash together. As you can imagine, his "whitewash" worked very poorly.
You are wrong on that. It's the same thing just different places use different words.
Just like "pencil lead" does not refer to lead. It refers to the thing inside a pencil. The composition of that changed over time but the name didn't.
There are plenty other fix terms like "gas" (gasoline) or "tin foil hat" (a hat made out of aluminium foil) which everyone knows what is meant by it despite the word(s) describing something different.
Nowhere in this thread anyone but you was taking about foils made out of tin metal.
You can also remove the paint and mirror polish the dish itself but that requires tools and it likely will not stay shiny for long.