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Viscosity, Finish Thickness, and the Magic of the Wet Mil Gauge

Jun 21, 2016 | Expert's Corner | 0 comments

Pro finishing shops maintain tight controls. Temperature, humidity, finish viscosity, air pressure, fluid volume, spray patterns – it’s a lot of factors that have to work together to produce the perfect finish, and measurement and control are the only ways to ensure a top quality result.

We tend to be a little more loosey-goosey in home shops. Controlling temperature can be a challenge in itself; as for humidity, we’re pretty much at the mercy of the elements. And while we may have to accept what Nature does, there are many elements of spray finishing that we can – and should – control if we’re looking for professional results.

Some are so basic that they shouldn’t need a mention, but they seem to just the same. One of these is proper lighting. Flat, featureless light veils all the details of the finish as it’s being applied. It’s a recipe for runs and sags. To see how the finish is behaving as you spray (or brush) it, you need low raking light across the surface of your workpiece, just like the lights they use at automotive paint shops.

A couple of shop utility lights will provide you with plenty of illumination and can be positioned exactly where required for best visibility. They’re also cheap and relatively no-hassle to store. So there’s really no excuse for woodworkers to continue to spray in the dark.

Now, why am I talking about lighting when the title of this article was about viscosity and wet mil gauges? Simply because if you don’t have adequate light to see what’s happening to your finish as it’s applied, there’s no point in adjusting viscosity or trying to measure film thickness. So please start with your lighting. It’s cheap and easy to set up and use, and is fundamental to you achieving good results.

Okay, now that we can see what we’re doing, let’s look at what we’re spraying and how we’re spraying it. A key measurement when we’re applying finishes is viscosity, which is how much a fluid resists the tendency to flow. Low viscosity = water. High viscosity = molasses in January. We determine viscosity by measuring how long it takes a specific volume of a fluid to flow through an orifice of an exact known dimension, and the tool we use is called a viscosity cup.
Ford and Zahn are the brands of two of the most commonly used cups. They will also have a number assigned to them, indicating the size of the hole in that particular cup. The Ford #4 and Zahn #2 are the cups you’re most likely to use, and results can be converted from one system to the other by using a conversion table.

The cups are easy to use. First, check the temperature of your finish and spray area. If you measure viscosity at 62˚ (about your lower limit for spraying) and spray at 75˚, you will need to spray lighter coats at the higher temperature because the finish will be thinner and will run more easily. One painting contractor estimated that a 10˚ increase in temperature will reduce viscosity by a corresponding 10 per cent. So make a note of the temperature when you make your measurements and be sure the finish has been stored in the same area so it’s at the same temperature. (It can take hours for a gallon of finish left in its can to acclimatize to shop temperature.)

Next, stir your finish very thoroughly. Lumps of pigment or a layer of solvent will throw your viscosity readings so far off as to render then useless.

Lower the viscosity cup into the finish until it オンライン カジノ ゲーム fills just to the brim. Lift the cup out of the finish and as it clears the surface immediately start timing, using the second hand on a watch or clock, or a digital timer. Stop timing just as the stream of finish breaks as it flows out of the hole in the bottom of the cup. Now compare the number of seconds on your watch to the time recommended by the manufacturer of the spray gun you’re using. That recommended time has been determined through testing to provide you with your best results.

If the time you measured is nowhere near the gun manufacturer’s recommendation, you may have to change the needle and nozzle set in your spray gun for a larger or smaller one. If the time measured is in the right range, per the manufacturer’s guides, then you can adjust the viscosity of the finish to “fine tune” it for best results.

Check the finish manufacturer’s recommendations regarding thinning and stay within their guidelines. For best results, thin minimally – perhaps 10% total volume – using the recommended solvent. For water-borne finishes your solvent is water, オンライン カジノ ゲーム but you’ll achieve better results using part water and part “conditioner” such as Floetrol, DynaFlo or Target Coatings’ SA5 Retarder. What these conditioners do is help to extend the drying time and improve the leveling characteristics of the finish.

Now we’re getting into the ball park. We can see what we’re doing, the temperature and humidity are within acceptable parameters, and we have the viscosity of our finish matched to optimum recommendations for our spray equipment. Now let’s see what we can do to lay down an online casino even coat of finish.

Assuming we have the optimum needle and nozzle combination installed in our spray gun, the controls we have at the gun end of the hose are air pressure, fluid control, spray pattern and speed/distance of application. It’s a dance to get them all right at the same time. You want enough air pressure to produce a fine spray – not coarse droplets but not too dry, either – and this must be balanced with the finish flow rate, again to give the オンライン カジノ correct volume of finish to produce a fine spray.
When these items are in harmony, you control the application of finish by varying the distance of the gun from the surface being sprayed, and the speed at which you move the gun. And this is where the Wet Mil Gauge or Wet Film Thickness Gauge comes into play. The gauge is about the size of a credit card and looks sort of like a mini toothed trowel used for applying tile cement.

The notches down each of the four edges are each 0.0001” (one thou or one mil) longer than the next. When pressed onto a wet finish the gauge leaves a series of marks (which will usually level out) that allow you to measure the thickness of the finish you’re laying down. Manufacturers will usually have a recommended mil thickness to be sprayed for the finishes they sell. Either by testing with the gauge as you spray, or by using the gauge as a training tool to learn what a certain film thickness should look like, you’ll quickly increase the consistency of your work.

When getting used to a Wet Mil Gauge, first try spraying horizontal surfaces. That way you can concentrate on your spraying without having to worry about runs and sags. With some practice under your belt you’ll have a good “feel” for spraying the right thickness of finish and will be able to spray vertical surfaces just as easily as horizontal.

You’ll remember I also mentioned that there is a spray pattern control on the gun. This can change the shape of what’s being sprayed from a tight circle to a broad fan, but as it does, it changes the volume of finish being directed into a specific area.

Consider this: if you spray an elliptical fan pattern that’s about three inches wide by 16 inches long, you’re spraying an area of about 38 square inches. In contrast, a spray pattern dialed down to a circle about three inches in diameter means that you’re pumping the same volume of finish into an area that’s only slightly more than seven square inches.

That’s about five times the amount of finish per square inch that was sprayed in the fan pattern and it will likely cause you serious problems if you don’t make adjustments to your fluid and air pressure controls. Using the Wet Mil Gauge as you make these adjustments helps you to determine quickly and accurately how to set the gun in order to lay down the proper thickness of finish.
Knowing that a couple of quick turns of the gun’s controls will keep your finish thickness constant as you change from inside corners to outside edges and flat panels can be a real boost to your finishing confidence. Dial down to a small spray pattern, reduce your pressure and fluid volume, and finish the inside corners, edges and details. Then increase the size of the spray pattern and adjust the volume of finish and your application speed to start spraying the larger panels.

For best control, keep a couple of sheets of cardboard handy. Make the changes to your gun’s settings, spray a test pass onto the cardboard, check it with the Wet Mil Gauge, and then get back to work on your project, knowing that the amount of finish being sprayed will be consistent from one pass of the gun to the next.

What’s the best thing about the Wet Mil Gauge? It only costs about three bucks. How can you lose? And while the gauge may look about as impressive as a plastic drill gauge giveaway from a tool show, it’s really a precision instrument and should be treated like one. Fortunately, about all that means is wiping it clean after use and protecting it from bangs and knocks so the one mil difference from one tooth to the next stays accurate. What more can I say?