Thermocast Sink

We bought our house new about 4.5 years ago. All the appliances are black, but the kitchen sink was the shallow contractor grade steel one. So we bought a nice Thermocast black sink from Home Depot. Within a few weeks the glossy shine was gone on the interior portion of the sink. The upper surfaces are still pretty but the inside is dull, and looks like the polish has rubbed off. Anybody know what I can do to remedy this?

You might have to start buying your appliances at a plumbing supply if you want them to last.

Two possibilities come to mind:

1. Do you have a dishwasher, or do you do your dishes in that sink. I'm thinking the problem is that your cuttlery and the bottoms of your dishes, are scratching the smooth finish of the sink. After all, it's the cuttlery that's going to fall to the bottom of the sink as you wash dishes.

You see, hardness is an extremely important quality in any material. The harder something is, the less quickly and easily it's damaged by things scraping or rubbing against it. The reason why stainless steel sinks are so popular is because stainless steel is considerably harder than mild steels, and therefore doesn't scratch easily.

I'd check with the manufacturer. If they don't specifically tell you not to do dishes in the sink, then it may still be under warrenty, and you may be able to get a replacement sink gratis.

2. Do you use bar soap to wash your hands in that sink?
The difference between a "soap" and a "detergent" is that soaps are made from animal fats and/or vegetable oils, whereas detergents are synthetic products, and don't contain any animal fats or vegetable oils.
As a result, real soap has fatty acid chains in it that can become insoluble in water. (which normally happens when there are hardness ions in the water). When real soap (dissolved in water) comes into contact with hardness ions, the hardness ions neutralize the polar group at the end of each soap molecule, effectively neutralizing it's apparant charge. When that happens, the soap molecules loses it's solubility in water cuz it's the polar group at the end of the soap molecule that's attracted to polar water molecules. The result of soap losing it's solubility in water is called "soap scum".
This is why you normally never get soap scum ring forming in your kitchen sink, but you do in your bathtub and bathroom sink. It's cuz you normally only use real soap in the bathroom. In the kitchen sink, you normally only use dish washing DETERGENT. Detergents are synthetic and can be formulated without a positive or negative polar group at the end of the detergent molecule to make it soluble in water. "Amphoric" detergents (meaning they don't have a polar group at the end of the detergent molecule, but something else, simply can't lose their solubility in water, and so can never form a soap scum ring. But, if you're using real soap to clean hands and other stuff in your kitchen sink, then that dullness may simply be soap scum.

Phosphoric acid is the active ingredient in many bathroom cleaners because it cuts through soap scum like a hot knife through butter, but won't attack chrome plating. Phosphoric acid is also found as the active ingredient in many toilet bowl cleaners, so you can use a phosphoric acid based toilet bowl cleaner to remove the soap scum rings in your bathtub. If you suspect that there might be soap scum in your new sink, I'd try a bit of phosphoric acid based toilet bowl cleaner to remove it.

Great response! Thank you! I emailed Thermocast sales support with basically the same info as my post. This is the response:

Thank you for your e-mail. To restore the luster to your sink we recommend using just plain bleach and water or Clorox cleanup. Then follow up with a liquid based auto polish such as turtle wax, Maguire's cleaner wax or an actual auto "rubbing/buffing" compound. Do not use soft scrub, comet, ajax etc.. it will ruin the finish, if the above remedies do not suffice please contact us back.



Nestor- We don't typically "do" the dishes in the sink. However, the plates, silverware, etc do sit in it between loadings of the dishwasher. We also will clean the sink thoroughly every couple of days, which means using the hand scrubber soap dispenser spongy thing. Which does have an abrasive pad on it. Do you think their suggestion of using auto polish or wax is a reasonable option? Won't it just wash off after a couple of days? Maybe that is the solution you use while you are showing a house you are trying to sell?

Brandon:

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

all squared.

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.

The sink must be some kind of plastic or polymer, when you warm up the sink with hot water and the dump a bunch of cold water in it might crack or break, if you get it hot with a pot it might warp or melt. check with the manufacturer on all the info for the sink.