One of the most widely used technologies for applying coatings has been referred to by a number of terms including a wire-wound rod, coating rod, metering rod, Mayer rod, Meyer rod, among others. It's a very simple device, inexpensive, very accurate, durable and easy to use. No wonder it continues to find wide application more than a century after its invention.
Invented in 1905 by Charles W. Mayer (so "Mayer" is the correct spelling), the coating rod is brilliantly simple: take a straight metal rod and over its length tightly wind a uniform layer of wire around it. While a hundred years ago, iron rods and wire were prone to rust, modern materials like stainless steel have assured the Mayer rod an enduring role in many industries.
The Mayer rod is most suitable for applying low to medium viscosity coatings. Fortunately, for our purposes it really isn't a significant limitation.
Before we can use it we've got to have it on hand. It's not difficult to find vendors offering coating rods for sale--a quick look on the web will show quite a few suppliers. In fact you'll see a number of variations on the theme, as well as rods pretty closely resembling the original designs, albeit made with better materials today.
Usually you'd need to request a quote after consulting the supplier's data sheet specifying rod diameter and wire thickness, factors that determine the qualities of the film the rod would "meter". The rod's coating parameters are often given as wet film thickness or dry weight of coating as a percentage of coating contents. Obviously, smaller diameter wire will produced thinner coatings. As a starting point, a rod wound with 50 mil (~1.25mm) wire produces a moderately thin coating. Naturally requirements will vary according to the artist's purpose.
What would it cost? A few years back, a 24 in (60 cm), 52 mil stainless wire-wound rod cost ~$100, not too bad for a tool that with proper care is usable for an artist's lifetime.
So you're thinking, "how do I know what I'd need?" Good question. It may take some experience, and likely repeated trial and error, before determining what's going to work for your needs. Who could afford it? Probably very few indeed.
But the good news is that I'm about to show you how you can make your own quite adequate coating rods, and rather inexpensively to boot.
Making your own coating rods:
Step 1. Of course you'll need a metal rod with the right properties. The crucial specifications are the rod's composition, diameter, roundness, straightness and length. Stainless steel (there are several grades, but not crucial here) is the best material. A diameter between 0.5 and 0.75 in (12 to 19 mm) will work well.
Roundness and straightness can be tested on a thick pane of glass by rolling the rod in different directions. There shouldn't be "gaps" between rod and glass surfaces, nor resistance to rolling (indicating lack of roundness).
The rod's wire-wound part needs to be at least 5 cm longer than the width of the stock to be coated. Since a longer rod can always be used to coat pieces smaller than maximum, a reasonable policy is having rods large enough to cover the largest surface you'd anticipate coating. Be sure to leave some length (~5 to 7.5 cm) at both rod ends to provide "handles" beyond the wire-wound portion. Artists typically employ rods with an overall length of 24 to 36 in (60 to 90 cm).
Step 2. Now that you have the metal rod, you need to apply the "wire" winding to it. Real metal wire would be tough to work with, so instead we're going to use a much more tractable material, namely Nylon. What you need is used in fishing, hence sold in shops selling marine or fishing equipment. It's sold as "leader line" in very heavy weights, rated at 100 lb. or more (>= 40 kg). This kind of line will specify its diameter as 1.0, 1.2, 1.3mm and so on. One of these larger thicknesses will be suitable--the weight rating is really irrelevant to our purposes, so look for the line's diameter. Just don't expect sales people to understand what you are doing--they inhabit an entirely separate universe.
Make sure to get a long enough spool of the leader line. For our purpose it's got to be one continuous piece wound around the metal rod. A 30 in (75 cm) winding on a rod (as specified above) requires at least 10 to 15 yards/meters of line. Far better to have surplus than not enough. And who knows, you may want to make another one later on.
Step 3. Start the winding process by drilling two holes through the rod, about 2-3 in (5-7.5cm) from the ends. Use a bit large enough so that the line will slip through the hole without difficulty. Optimally you'd use a drill press to make the hole purpendicular to the rod and centered, that is, the hole ideally traces the diameter of the rod, and even better if the holes align nicely, but that's not really essential. If using a hand-held power drill, well, just do the best you can.
You'll probably need to get drill bits made for hard metal; these are usually cobalt-tipped. You'll only need to invest in a few of them, so the expense should be modest.
Step 4. Once the holes are drilled, the fun really begins. The trick is avoiding tangles in the leader line as winding proceeds. Make sure you know how the line is packaged. Usually (but not always!) it comes on a spool. Inspect it carefully so it can be unspooled without getting out of control or jamming up as you apply it.
Feed a length of the line through one of the holes you drilled, having 3 inches (8cm) showing out the other side. Anchor it with a short length of gaffer tape, but not blocking the hole through which you inserted the leader line.
Now start winding. The idea is to keep the coil tight, with no (or minimal) gaps between the horizontal "row" of threads laid down. This requires that the winding progresses under some tension that must be continuously applied. If the tension is released, even momentarily, the whole thing will immediately "spring out" and a mess to deal with.
It can be helpful to "lock" part of the winding in place periodically as you go. You can do this by using some gaffer tape to stabilize the sections already wound--just make sure not to wind the line over any part the tape, so the tape can be removed later when it's no longer needed.
Step 5. When the winding reaches the hole at the end of the rod opposite where you started. It should be nice and tight, without looseness where the line contacts the rod, providing a solid feel to the coil. Keeping tension on the line, use some tape to secure the last of the coil to the rod, like the "lock" described above. Then the tension can be released since the "lock" is in place. Cut the line some distance beyond the end of the coil, and insert the free end through the hole and pull it tight, taping it down so the tension is retained with the other "locks" are released.
Step 6. Finishing the coating rod. Use waterproof tape to permanently secure the free ends of the the leader line. Make sure there aren't big "lumps" of tape/line at the ends because protrusions might impede using the rods during coating applications. Using water resistant tape is important because the rods will need to be rinsed off frequently. Never let coatings dry/harden on the rods--it's guaranteed, they will be difficult to restore once coatings get embedded in the grooves of the winding.
Congratulations! The coating rod is prepared, apply some mild detergent, hose it off and dry thoroughly. Now it's ready to use!
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