There is no disputing the fact that polyurethane is not actually crystal clear as the name Behr PREMIUM PLUS WITH STYLE® Crystal Clear Water Based Polyurethane No. #780
would suggest. Based on testing performed at the Behr Color Laboratory, two rolled coats of the polyurethane typically reduced the Blue by one or two points. If you are striving for a true neutral gray screen then this can be compensated for by selecting a gray tint that has the red lower than the green and blue. A good example would be Sherwin Williams "Gray Screen". It has an RGB of 199 203 203. The polyurethane would change this to 199 203 202, 199 202 201, or worst case 200 201 200. So in fact it might actually take a very near neutral gray paint and make it even more neutral.
One other consideration regarding a polyurethane top coat is that it does seem to have a softening effect on the image. What I have observed is the light diffusing effect of the polyurethane tends to blur the black lines between the pixels. For some this may mean the image is not as sharp while others may see it as a way to reduce screen door effect and create a more film like image. Either way you should be aware that this does occur. The introduction of some pearl to the polyurethane seems to eliminate this softening.
I would not recommend using a simple polyurethane clear coat on a screen if you will be getting a 1080p projector.
The effect the pearl additives have on color is not quite so well known at this time. Some have reported a blue push with pearl clear coats and some have measured a decided red push. Due to some confusion I am not entirely sure what has been tested though.
Originally I presented some pearl clear coats that were made using Folkart Metallic White Pearl. I found that one or two 2oz. bottles of this white pigment and pearl flakes added to Behr Matte Polyurethane produced a further gain boost without any untoward reduction in viewing cone. At that time the designations 1xPearl and 2xPearl were coined to denote one or two bottles of the Folkart Metallic White Pearl were added to a quart of Behr polyurethane. This naming convention has never changed. I have always suggested two rolled coats and all my testing was done with two rolled coats.
I happened upon a pearlizing medium at Michaels one day and realized that there was an even cleaner source of pearl flakes readily available. I had been resisting the temptation to introduce automotive pearl powders and what not since I was striving for the best I could do with materials the average North American could pickup on the way home from work. I tried the Liquitex, Winsor & Newton, Delta, Decoart, and Folkart iridescent/pearlizing mediums mixed into Behr matte polyurethane. While the Liquitex and Folkart products seem to be very similar the Folkart came in a more suitable sized bottle and was quite a bit cheaper. The Folkart Pearlizing Medium also did not seem to introduce as much unwanted sheen as the Liquitex product. I found the Delta product had very little pearl in it, while the Decoart product had a noticeable red push to it. The W&N medium had much larger and very silver looking flakes that introduced a noticeable sparkling. So I settle on the Folkart Pearlizing Medium as the best performing as well as the most cost effective choice.
The pearlizing medium was a much better source of pearl flakes because it did not introduce the unwanted warm white pigment that the Folkart Metallic White Pearl did. It was my intension to only present the Folkart Pearlizing Medium until people in the USA started reporting difficulties finding the product. Therefore in the end I presented all three methods of implementing a pearl clear coat. The original 1xPearl, 2xPearl clear coats and the purer Pearlizing Clear Coat.
Hopefully that recap of the history of my pearl clear coats investigations will clarify that 1xPearl has nothing to do with the number of coats or the recommended Pearlizing Clear Coat.
I would speculate that the 1xPearl and 2xPearl clear coats will have more effect on color balance due to the warm white pigment they introduce. It was my hope that the pearlizing clear coat would simply introduce a gain boost and light diffusing effect in the polyurethane. Optical Texture was the term that was coined at the time. I have no idea how much any of these pearl clear coats effect the color balance. I do know from extensive side-by-side viewing comparisons that any shift was not immediately apparent. I do however expect that there is some effect and that it can be measured. I also know that it can easily be compensated for by adjusting the base gray.
One very significant difference I noticed between the pearl clear coat approach and the metallic mix approach is that while you can see the pearl flakes in the clear coat when inspected very closely, you do not get the same shimmering effect in the image that I noticed with RS-MaxxMudd and my derivative of it using only Folkart Metallic White Pearl.
I have the means to perform a proper and rigorous set of tests of these pearl clear coats to determine how much relative shift there is in the RGB values. I also have more than enough sample cards with exactly the same gray base applied to them. The absolute RGB of this gray base is not relevant to determining the relative change in RGB due to the three different pearl clear coats and the number of coats applied. As time permits I will make up the following samples and measure the relative differences in RGB readings.
- BG (Behr UPW 1050, Qt 6LB + 3YO)
- BG + 1 Rolled Coat Behr Polyurethane #780
- BG + 2 Rolled Coat Behr Polyurethane #780
- BG + 3 Rolled Coat Behr Polyurethane #780
- BG + 1 Rolled Coat of 1xPearl Clear Coat
- BG + 2 Rolled Coat of 1xPearl Clear Coat
- BG + 3 Rolled Coat of 1xPearl Clear Coat
- BG + 1 Rolled Coat of 2xPearl Clear Coat
- BG + 2 Rolled Coat of 2xPearl Clear Coat
- BG + 3 Rolled Coat of 2xPearl Clear Coat
It should then be possible to pick specific brand name paint tints that will have an equal but opposite color profile. The two opposite color shifts will cancel out just as they do with pigments in the neutral gray paints.
It should be apparent that a gray base with a clear coat is a two part application where the two layers should be tailored to work together. The point is that while I have not observed any seriously detrimental color shifting when a pearl clear coat was applied to an existing gray base, it is not the ideal and will no doubt produce a measurable change even if it is not terribly noticeable to the eye. The point being that you should be aware that if you apply a poly or pearl clear coat over a true neutral gray that the RGB will be shifted and possibly more than the acceptable 3 or 4 points.
Let me restate again that my goal is now and always has been to develop the best DIY Screen Paint that I could using readily available products from Home Depot, Michaels, Wal-Mart, and your local fabric store (or similar retailers). I am not suggesting these are the "BEST" solutions. There are other very talented people working on paint solutions employing aluminum powders etc and they have spectrometers and university degrees and know a lot more about color theory than I do. If the very best possible DIY Screen paint is your desire then seek out those individuals and keep watching because they will be presenting some very interesting solutions in the future.
On the other hand, if you just ordered your first projector and it is arriving next week you probably want to throw something together in the way of an interim beginner's projection screen. Then you are the intended audience for my simple DIY painted screen solutions that only employ wall and craft paints.
I have had a chance to do a direct comparison of a Pearlizing Clear Coat sample panel to my buddies Da-Lite High Contrast Matte White retractable screen. The Da-Lite HCMW is a gray base with a pearl clear coat. It also employs a combination of sheen and surface texture. it was difficult to tell them apart right down to being able to see the pearl flakes on close inspection. I'm sure there would be differences if measured with a spectrometer and proper gain curves were plotted. That should qualify what I consider to be an adequate DIY Screen Paint solution.Pros:
I should mention there are some positives to applying a polyurethane top coat or pearl clear coat. The first advantage is that the surface will be more durable and washable. The other advantage is a gain boost without any serious issues with hot spotting or viewing cone reduction.
A poly or pearl clear coat is not at all necessary. I have had very good results with using matte finish paints alone. I only suggest these top coatings if you want to add a little more gain using stuff you can pickup locally. If you do not want to be bothered with top coating then stick to a good quality matte paint such as Behr's Flat Enamel or better yet the new Behr ULTRA Exterior Flat paint.