If the extension is chrome plated, then it was probably electroplated. While it might seem as if it would be easy to plate the inside surface of the tube at the same time the outside was plated, in reality electroplating is not this simple. Electroplating the inside of a long, narrow tube is difficult because an electrode must be placed inside the tube. I.D. plating is also made difficult because the anode area is so much smaller than the surface being plated. I am sure the chrome-plated tube looks cool (on the outside), but it was a bad choice because even if the inside of the tube were plated, it would probably be bad plating.
As you discovered, there are other choices for extension wand materials and finishes, such as galvanized steel or (unfinished) aluminum.
Your poly pipe fix won't stop the internal rusting (in fact, it will probably accelerate the corrosion), but at least you'll keep the rust out of the nozzle for a while.
A lot of items are really only intended to be used once, with the rationalizing being that the average consumer would purchase the product for a specific use and then not use it again. Superscope did that with their cassette recorders back in the early 1980s. The mechanical parts were made of cheap plastic and they would wear out with any amount of use. The explanation that I was given was that "they are intended for somebody who buys it, uses it once, then puts it in a drawer and does not use it again".
That kind of thinking is more common today than ever, and it shows up as a constant string of products to be described in this column.
This reminded me of the last time I actually refinished my own deck--a ton of backbreaking work even with one of those sprayers that's built somewhat similarly to this one. Of course, I bought the cheapo plastic one, and of course, no one told me it would clog no matter how I tried to clean it. So yep, when I went back to finish the rest of the project a few days later, the $40 sprayer was plugged and could not be fixed. I was supposed to go buy another one. I did the rest by hand, cussing all the way. I later learned this is standard--what nonsense!
It is possible that the tube was originally designed to be plated internally. The product was then shipped overseas for mass production at a low cost manufacturer. Someone there decided that skipping the internal plating would save cost and time on the production floor. The product was made and shipped to the US for sale hoping that no one would complain. Now the manufacturer is no longer a supplier, but the end company has too many in the distribution to change them out and the consumer loses. It has happened before and will happen again.
Good point, Dave2012. Yet at a certain point, I would think crummy products would begin to come with costs -- both in replacement and damage to the brand. I think we're seen quite a bit of this over the past couple decades and once-stellar brands have taken a giant hit on quality.
Ann, actually it is quite possible to clean the plastic deck sealer sprayers, I have done it a few times. After the first coat is applied and all of the sealer used up, run some mineral spirits through the spray tube and spray head. Use the cheapest spirits that you can buy. And you can save them in a sealed jar for cleaning the second time. But running a cup or so through the spray head , wand, and control valve does a fairly good cleaning out. Then store the nozzel in a small jar submerged in mineral spirits. That keeps any residue from hardening. After the last use and cleaning you can run some cheap oil into the sprayer. Even used deep-fryer oil, if you filter out the crumbs. Then rinse it with detergent and water before using it the next time. The same approach also works for my $110 Binks sprayer.
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