The Fine Points Of Viscosity
From Paul Snyder, technical advisor Finishing Forum:
The right viscosity depends on the type of spray gun you're using, the size of the needle/nozzle in the spray gun, and the coating. If you have one spray gun and one needle/nozzle set, then you'll have to thin everything you spray to match your spray set-up. The actual viscosity depends on the set-up you have. For thicker material (higher viscosity), you'll want a larger needle/nozzle and vice versa. Related web page: Measuring Viscosity
From contributor B:
With my setup, a HVLP gravity feed gun with a .050" needle/nozzle and shooting MLC MagnaMax precat lacquers I use about 25 seconds with the #4 Ford Cup. I can push it up to about 30 seconds, but it really likes 25 better.
From contributor C:
The best way to tell what viscosity to use is to spray the finish on and see how it looks as you apply it. The viscosity testers are only a guide line. As you spray more and more it will be like second nature. You will be able to tell the right viscosity just by how it drips off the stir stick.
From the original questioner:
I spray lacquer mainly with a HVLP gun. I have several tip sizes, I like the 1.3 best and have used a 1.7 also. I know there are too many variables here to really nail it down but I was hoping to see a pattern. Some of the lacquers say "no thinning required" but they are way too heavy for any of my guns so I usually thin about 50%. Does this sound like too much to you? And does it really matter if you thin more?
From contributor D:
Yes, it does matter as it's a waste of material and VOC. I normally use 15 seconds as well but to achieve this I only thin 15% with lacquer thinner and then, since I live in a very hot climate, Phoenix, AZ. I add 4 oz of MAK per gallon of precat (Valspar Valtec). This gives great flow. There’s no way anyone should need to reduce lacquer 50/50 in this day and age.
From contributor E:
To the original questioner: Yes to both questions. 50% is way too much for a topcoat and will affect the durability. I have tried over the years to manipulate my pressure, gun settings, etc. to reduce the amount of thinning to comfortably spray. I use a Ford cup on a new batch of spray to get a feel for the viscosity of that particular batch, which will vary from one to another. However, like someone said, after a while you kind of get a feel for how it will spray. A couple of months ago I switched my topcoat and in doing so, I had a conversation with my coatings rep. I was talking with him about viscosities and such and I said something like "... they all say to avoid thinning but I don't know of anyone who sprays right out of the can - I don't think you can." He kind of chuckled and said "I never thin my topcoat, I always spray it right out of the can". I switched to MagnaMax and I think it would be difficult to spray without thinning, especially the way it sprays, but I think thinning should probably be done as little as possible for the best performance. You do get a feel for things over time and can tweak settings and such according to temperature and all those other quirks that affect how your finish is laying down. I have always just tried to strike a balance between good atomization, pressure settings, overspray and thinning. The more I spray and read these forums, the more I realize how much I don't know.
From contributor C:
I spray almost everything straight out of the can. When you thin it you change the performance of the finish. I have had to change my method of spraying though. I use an air-assisted for almost everything. Just got a visit from the air cops today and reminded me that everytime you thin it you stand a chance of going over the limits of the rules laid down by the air cops. And that the VOC limits are being lowered again in California in the middle of this year. I only thin with acetone because it's exempt. The problem with HVLP guns is that they don't atomize the finish enough to shoot it full strength.
From contributor F:
The answer for you poor devils out there in low-VOC land is to use a heater. There are many models of lacquer heaters available. Another part of the viscosity checking is temperature. The higher the temperature, the lower the viscosity - so you can thin your material by heating and not affect the coating integrity. (Unless you boil it or do something stupid) That is another good reason to get in the habit of checking viscosity. One day I may need 10% thinner and on another really hot day I may not need any - even out of the same can.
From contributor G:
There are 3 parts to good finishing: Product, Technique, and Equipment. If something goes wrong, our supplier will be quick to blame your equipment, and your equipment salesman will blame your product - neither wants to say that you have bad technique!
As a result, you need to understand what effect your equipment has on the product you are spraying. If you bought a good setup, then you will have great tech support. Call and ask them that you are using a Ford cup, and what kind of viscosity you should have with the setup (needle size, air cap, etc).
Then, ask your finish supplier for their recommendations. "No thinning required" may be meant for painting contractors who use an airless. Most of the precats that I know of recommend no more than 10%. When you thin 50%, you will need twice as many coats to get the film thickness you need. Like these guys have said, there are many variables to viscosity.
As for me and my equipment, I get a needle setup for the viscosity of the material that I will be spraying. Then I set the needle so I can pull a full stroke. And on my last coat, I thin 10% and lay out a great finish. Sounds like you need to talk to your suppliers a bit more and go from there.
From contributor H:
It's all in your knowledge of dialing in your equipment and your technique and hopefully if you’re competent enough to know this much you should be up on your product knowledge as well. There are one or two other things that will get you from good finish jobs to great finishes and that is adjusting for the weather conditions and also the jobsite or shop conditions as well. Also the idea that buying great spray equipment will go hand in hand with great tech support isn't always a given. Heating your material is a great way to lower viscosity of a coating but will only cause you more headaches with the VOC compliant finishes based on acetone, You'll just wind up dropping in retarder to slow down the drying rate. I have never used a viscosity cup only because of all the reasons that have been said although it would have came in handy when I was starting out to get myself at least into the ballpark. I also learned by the old stir stick drip rate scientific tool method. Another problem we have in CA is the fact that if you thin too much and get caught by the air cops, we run the risk of some pretty hefty fines.
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