I don't sell houses, I rent apartments.
But, fundamentally, the problem here is that Thermocast's suggestion to use an automotive polish in the sink to restore the gloss is an admission that the material their sink is made of is being scratched up by the cuttlery and dishes being stored in the sink. But, truth be known, even the best stainless steel sinks (like Franke) will get scratched up by sharp Henckel knives you put in them. The reason why, of course, is that you can use a harder stainless steel if you're making a knife than you can if you're making a sink. A knife just has to be sharpened; it doesn't have to be soft enough to be stamped into a radical shape like a sink without cracking or even shattering. So, by necessity, knives are made from harder stainless steel that will keep a sharp edge longer than sinks, which have to be made of soft enough stainless steel to be stamped into a radical shape. So, knives in ANY sink are gonna scratch up that sink. But, owners of Franke sinks will claim that the scratching up of their $800 kitchen sinks adds to the "patina" of the sink, which they think is a good thing. I call it "scratches", they call it "patina".
The best automotive "waxes" are made from Carnauba Wax, which is the same stuff people used to use on their hardwood floors before the Bayer Company (the Aspirin people) patented the first "polyurethane" in 1956. Carnauba waxes seem to stand up well to water, but they're not very hard, so the Carnauba wax on your sink is gonna scratch up even faster than the sink did. So, you're going to have to periodically wax your wax to restore the gloss. If you ever want to remove the Carnauba Wax, you can use Ammonia to strip it off your sink.
The reason why using a transparent coating (like a car wax) on your sink will hide the dullness and restore the gloss is because of something called Fresnel's Law. Fresnel's Law says that the amount of light reflected at the interface of two media with different refractive indices is a function of the refractive indices on each side of that interface. Where the incident light is perpendicular or nearly perpendicular to the interface, then Fresnel's law can be simplified as follows:
Fraction of light reflected from interface equals:
the difference between the refractive indices on each side of the interface divided by
the sum of the refractive indices on each side of the interface
That is: Percent reflected = ((RI1-RI2)/(Ri1+RI2))^2
Fresnel equations - Wikipedia, the free encyclopedia
So, even though Carnauba Wax and air are both transparent, Carnauba Wax is a solid and as such will have a higher refractive index, which will be closer to that of Thermocast plastic. To explain why the Carnauba Wax will hide the dullness, let's do a simple calculation. The refractive index of air is about 1.00 and let's presume the refractive index of Carnauba Wax is the same as Plexiglas, or 1.51. And, let's make a scientific wild a$$ed guess and say the refractive index of the Themocast plastic is 1.8 say.
Light is currently reflecting off your Thermocast plastic. On one side of that reflecting surface you have air (R1=1.0) and on the other you have Thermocast plastic (R2=1.8), and so according to Fresnel's Law for near perpendicular incident light, the fraction of light reflected from that interface will be (0.8/2.8)^2 or 8 percent. The remaining 92 percent of the incident light will enter the Thermocast plastic and be absorbed. But, that 8 percent reflected light is enough for your eye to see that the surface of the sink is dull.
Now, let's put a coat of Carnauba Wax over that Thermocast plastic. The amount of light reflecting off the smooth Carnauba Wax will be: (0.51/2.51)^2 or 4.13 percent. The remaining 96 percent of the light will enter the Carnauba Wax. The fraction of incident light reflecting from the Carnauba Wax/Thermocast interface will be:
96 percent of (0.29/3.31)^2 or 0.74 percent.
So, those sink scratches will still be under the Carnauba Wax. However, because of the presence of the Carnauba Wax, now 4.13/(4.13 + 0.74) or 85 percent of the reflected light is coming from the smooth air/Carnauba Wax interface and only 15 percent of the light you see is coming from the scratched up Carnauba Wax/Thermocast interface. The result is that the scratches are much less visible and hence noticable, and the smooth surface of the Carnauba wax is most of what your eye sees, making the sink look glossy again.
You could do exactly the same thing with a clear grout sealer meant for sealing the grout in showers like Tile Lab's "Gloss Sealer and Finish" available at Home Depot. I really don't know whether this product or Carnauba Wax would stick to the sink better, but I do know that the Gloss Sealer and Finish is highly water resistant. I really don't know if Carnauba Wax is more or less water resistant than "Gloss Sealer and Finish". The latter can easily be removed with Tile Lab's "Heavy Duty Cleaner & Stripper" also available at Home Depot, or with acetone.
Personally, I'm a great believer in practical solutions. The reason why stainless steel sinks are so popular is because they're strong enough not to dent if you put a 20 pound turkey in them, they don't rust, they're fairly hard so they don't scratch as easily as most materials, and if you're prepared to live with the development of a magnificent "patina", they'll last darn near forever. If it was me, I'd have probably left the cheap stainless steel sink in.
But, I'd say your best bet now would be to try various clear coatings over your Thermocast to see which one lasts the longest, and I'd probably start with the car wax. A "Wiping Polyurethane" meant for furniture would be harder than most finishes, but you'd have to strip it off before other coatings would stick properly (and that could be done with a paint stripper). An acrylic grout sealer might not stick all that well to metal, but you could apply multiple coats without stripping the previous coat off.