Care and Repair of Acrylic Aircraft Windows
Story and Photography By Ron Kilber
WHEN MY AIRPARK NEIGHBOR returned from an FAA-sponsored maintenance seminar (where the instructor claimed he hadn't yet run across acrylic that couldn't be repaired), I just had to find out more about that. Wow! If that's true, I realized that I might avoid replacing the hazed windows on my own Cessna 150. More than once, when landing at dusk, I had to go around and land downwind just to keep the sun's glare off the hazed windshield.
So began a repair odyssey that started with anticipation and excitement, then deteriorated to discouragement and disappointment after I couldn't achieve optical clarity not much better than when I began. Then, like Superman arriving out of nowhere, an expert showed up at my hangar to set me on the right path. Now I've got an airplane that I can see from -- together with a wide smile with a lot of saved money still in my pocket.
Neither an expert of acrylic nor its repair, I really didn't even know the real truth about aircraft window care and maintenance -- let alone restoration -- until I got this story assignment. Much of what we learn as pilots comes from other aviators who've shown us the ropes over the years. For example, more than one experienced pilot has advised me to use car wax to fill scratches on acrylic. Even my Owner's Manual recommends wax to hide minor scratches, though no mention about the type of wax. Any pilot or owner could assume anything from bee's wax to petroleum-based wax, never realizing that damage could be taking place that wouldn't be visible until years later. Now I know better -- and why. And there'll be old bloats with lots of egg on their faces after learning what I've discovered from the experts.
It should be inspire you to know that even an acrylic neophyte like me actually managed to not only repair his Cessna 150 windows but, more importantly, I also learned a lot about acrylic care and maintenance. Even if you never tackle a window restoration project on your own, you'll at least find out what I did, not to mention the real truth about how to take care of your aircraft windows.
After calling the FAA in Scottsdale, Arizona, I managed to chase down the seminar instructor, an employee of Bombardier Incorporated in Tucson. From there I learned of other industry experts, as well as companies, in the acrylic-repair business. While there are many products and kits out there to serve this market, the experience I gained for this project is limited to the Micro-Mesh and 3M systems. In fairness to all, more than one expert told me that virtually any acrylic repair system works, market share, customer service and technical support driving one's preference.
To get started, I ordered the Heavy-Damage Removal Kit from Micro-Surface Finishing Products, Inc., in Wilton, Iowa. This company has been in business for years, serving many military units around the world, as well as corporations such as Lockheed Martin and Boeing.
Selling for $68.75, the kit is one of six offered that removes everything from sub-surface crazing and gouges to scratches, halos and haze. For tackling less damage, there's a smaller kit priced as low as $21.50.
Especially useful for me was the technical information manual shipped with my kit. I learned, for example, that Plexiglas is a trade name for acrylic, as is Acrylite, Lucite, Shinkolite, etc. As far as I knew prior, each was a different plastic, not one and the same cloaked with different trade names. I also learned from my tech manual that other plastics, such as Lexan and Tuffak, are polycarbonates (of bullet-proof fame), which are tougher to attain the clarity as with acrylic (caution advised when restoring these). Finally, I learned that my repair kit works for fiberglass, painted surfaces and some resinous materials too.
EQUIPMENT AND TOOLS REQUIRED
While it's entirely possible to fix acrylic windows without power tools, after the job is all done, the old elbow might need fixing next. Otherwise, all that's required (besides the kit) is a random orbital sander, angle polisher and a high-volume air compressor capable of maintaining 120 psi continuously. A spray water bottle and a few all-cotton towels or rags round out the list.
Being a little nervous about using power tools on my trusty little Cessna 150 windows, I decided to take my neighbor's offer to practice on his motorcycle windscreen, which was severely gouged and damaged. Beginning with 1200-grit paper and working through all the prescribed steps, I managed (with beginner's luck) to put a big smile on my neighbor's face. As far as he was concerned, I had traded in his old windscreen for a brand new one. And he regretted giving away another windshield he believed was worthless and loaded with scratches.
With my new confidence and experience gained from the motorcycle, I began work on my Cessna 150. For window hazing problems, the directions say to begin with 1500 or 2400 Micro-Mesh disc. Wanting to be conservative, I started with 2400.
Using a pneumatic sander, I worked away on the rear left window, being careful to use enough water to prevent scratching and a pattern to remove plastic evenly. The water spray I used gradually turned to a milky-white substance. At first I thought I was removing 27 years of surface-contamination build-up. Each time I stopped and wiped away the white stuff, more always appeared as I worked on.
Eventually, the window surface became translucent, convincing me that the white stuff was actually released acrylic, made opaque from all the sanding. The whole process of using 2400-grit paper took me less than five minutes, including clean-up of the window for the following step.
Next the directions called for a 3600 Micro-Mesh disc, which I used until I could see symmetry throughout the plastic using a bright light from an angle. Once I was satisfied that this process removed all the 2400 scratches, I cleaned the window again. No more than five minutes here too.
Use of the Tuf-Buf polishing pad was next. Using a helping of Micro-Gloss for several minutes magically began to transform the window from a dull appearance to one of approaching optical clarity.
Micro Gloss is a one-micron, water-based compound specially designed for removing fine scratches, haze and swirl marks created by improper cleaning methods. It's a cleaner too, and can be applied by hand or power tools. The compound is even approved for F-16 aircraft canopies.
The last step involved more Micro-Gloss, only this time using a white foam sponge pad instead. Even more optical clarity was achieved.
To clean the pads between applications, I got rid of the water the same way dogs do. I spun the lambs-wool disc at a high rpm rate until it was bone dry. Be careful to avoid what happened to me. Don't run the pneumatic tool at full tilt to shed all the water. My pad came apart and launched into the air with the lambs wool still on it. Good old RTV (room-temperature volcanizing rubber) solved the repair problem, but I had to quit early so the mend could dry over night. The next day the pad was stronger and better than new.
Being a perfectionist, I wasn't exactly satisfied with the clarity of my rear window. It was certainly in better shape than before I started, but there are many airplanes at this airpark with much clearer windows. Wanting something approaching the same thing, I repeated all the steps again.
Still, in the end I made no improvement. Things were still the same. I even tried a third time using elbow grease alone (i.e., no power tools), but still I had no better luck. Now what?
At this stage I was discouraged and disappointed by the thought of my acrylic being too far gone for restoration purposes. I began to resign myself to the fact that I'd have to eventually replace all my windows.
Until, that is, I finally was able to speak to someone from the 3M company,. I had been playing phone-tag with them for more than a week. Luckily the representative lived only 90 miles down the road from me. Even better, he said he could stop by my hangar on his way to another appointment the following morning.
Dennis Brenna from 3M arrived the next day, complete with enough tools and material to fix the windows of all the F-16s at Luke Airforce Base. All that was needed was to turn on my air compressor for his pneumatic sander.
The 3M Finesse-it Plastic Repair System is similar to Micro-Mesh, using the same 5-inch disc size. The difference, of course, is in the manufacturing process that goes into fabricating the product. Also different is the labeling of each disc. While Micro-Mesh uses 1500 grit, 2400, etc. to name different sizes, 3M used 3 micron, 5, 9, etc. 3M doesn't have repair kits. Instead, each product or material is available individually from authorized distributors.
A SECOND GO
After carefully examining the left rear window I'd failed with, Brenna made the decision to start with a 15-micron cutter. Then he moved to 3-micron polishing paper. Finally he buffed his work using a polishing pad and Finesse-it finishing compound.
Like me, Brenna wasn't totally satisfied with the amount of clarity he'd achieved, but he didn't make the same mistake I made by giving up. Instead he began the whole process over again, this time starting not with the 15-micron cutter but 30 micron instead.
When finished, I was amazed by the tremendous amount of clarity he had achieved. Wow! Now I couldn't wait to do the rest of the windows on my airplane.
As Brenna explained it to me, the 15-micron cutter wasn't enough to remove enough material to make all scratches and pits vanish on the first go. But the 30-micron cutter was easily up to the task.
This begged the question: Would I have achieved the same amount of clarity using Micro-Mesh and a step courser grit? That answer came the next day when I restored the right rear window of my C=150. Yes, by starting out not with 2400 grit but 1500 instead, I feel I ended up with every bit as good a job as Brenna achieved with the 3M system.
THE MORAL OF THE STORY
When I began researching acrylic-restoration systems, I was cautioned by many experts that not all window damage was repairable. Had I not know this, like a blind sow stumbling across an acorn, stubbornness alone might have kept me at it until I figured it all out on my own. Certainly I would've faired better to achieve the clarity 3M's Brenna did. Instead, I resigned myself too early to the fact that my 27-year old windows were in the too-far-gone category. As I almost didn't find out, nothing was further from this truth. Luckily for me, 3M arrived and saved the day. Otherwise, I would've thrown in the towel and eventually spent some real money on new windows and installation.
THE 3M DIFFERENCE
The 3M process through me for a loop. One of the handiest features of the Micro-Mesh kit is the Velcro-backed material, which easily affixes to the disc of the pad on the random orbital sander and angle polisher. The 3M abrasives use both Velcro and self-stick fastening systems, while Micro Mesh is all Velcro-based.
After Brenna left and I continued on my own, I didn't have the adapter to receive the adhesive discs. And I wasn't willing to drive 100 miles to Phoenix to get one. So I improvised using every aviators stand-by: 100-mph tape. It sticks to Velcro just as good as it does to the fabric on wings. The 3M discs adhere to the smooth side of duct tape so well that whenever I wanted to remove the abrasive paper, the duct tape came off the Velcro first.
A NEW LOOK FROM MY WINDOWS
When I finally finished restoring all windows, I was satisfied with the optical clarity achieved. Interestingly, some of my airplane was done using the 3M system and some Micro-Mesh. Honestly, I can't tell the difference between the two systems.
CARE AND MAINTENANCE
After learning what the experts say about acrylic cleaning and care, we may never again let any line person touch our windows. I'm horrified whenever I recount the things that I -- and others -- have done to my windows over the years. It seems everyone, except the experts, have it all wrong. Not even my Cessna Owner's Manual gets it right.
First, the only one way to clean acrylic windows is to flush the surface several times with water. This removes dust and loose dirt. Stubborn spots can be loosened using thoroughly saturated cotton cloth or finger tips while running water over the surface. The window frame must be also be flushed and cleaned to remove the risk of moving dirt to the acrylic when wiping the edges.
Second, to finish cleaning, use an acrylic-designed product that removes rather than fills the damage. Products like Micro-Gloss or Finesse-it will fix the window with each cleaning. With old windows such as mine, they'll only get better with age.
THE DON'TS WHILE CLEANING ACRYLIC
Never use a circular motion when rubbing the window surface. Remaining dirt will cause random, round scratches, dramatically reducing vision and optical clarity when flying into the sun. Vertical patterns are less noticeable than horizontal.
Never wipe plastic that's dry. Dust and foreign particles will scratch the surface. Worse, wiping "dry acrylic" builds up a tremendous static charge, attracting even more dust and dirt.
Never employ a used cloth. Only fresh clean cotton should ever be used. Anything less will only have left1over dirt, debris and dust from previous usage.
Never use paper towels or cloth containing nylon or synthetic fibers, all of which are abrasive. Use only 100 percent cotton when cleaning or restoring acrylic.
Never use a cover on acrylic. Even 100 percent cotton can ruin a windshield after the wind chaffs it from the dirt that gets blown under the cover.
Never use any of the following to clean acrylic: gasoline, benzine, alcohol, acetone, carbon tetrachloride, lacquer thinner or glass cleaner. Each will damage the plastic in one form or another. You can't go wrong using water alone.
Never use a wax or filler unless it's specifically designed for acrylic applications. Petroleum-based waxes, for example, will attack acrylic.
Restoring my Cessna windows was a fun process. There's real reward from seeing the results of effort after just a few minutes. Given the high cost of custom acrylic windows and labor-intensive installation, we can't go wrong with at least giving self-repair a go. After all, we can always resort to replacing the acrylic after failing the task. Then we'll only be out the nominal cost of repair supplies or a kit. The reward of having a plane that can be flown into the sun again is well worth the try. With continued cleaning and care, I feel my Cessna 150 windows will eventually be as good as they were when my airplane came off in the assembly line in the 70s.
Good luck with your acrylic windows!
Photos to appear later.
# 1. This Boeing Stearman windscreen, made from 7/32" safety plate (solid glass), is as clear today as when first manufactured during WWII. When the aircraft industry began using plastics for windscreens, so began care and maintenance headaches for owners and their mechanics.
# 2. Severe hazing, exterior view, Cessna 150 rear window. This damage occurs after many years of exposure to the elements and/or improper care and maintenance. Careful examination with a magnifying glass reveals thousands of scratches and pits, which diffuse and distort the light passing through the window.
# 3. Severe hazing, viewed from the baggage compartment, Cessna 150 rear window. The same damage appears different in full sunlight than in shade, which in this case is cast by the aircraft flaps fully retracted.
# 3. Mild hazing, viewed from the cockpit, Cessna 150 windshield. This type of damage can make landing into the sun extremely dangerous, distorting visibility to white-out conditions, making even large objects on the runway impossible to see.
# 5a, 5b & 5c & 5D. Severe crazing, exterior view, Cessna 150. This damage occurs from prolonged stress to the window, especially where it bends to conform to airframe lines.
# 6. The Heavy-Damage Removal Kit from Micro-Surface Finishing Products, Inc. One of three packages offered, this kit removes everything from sub-surface crazing and gouges to scratches, halos and haze. Especially handy is the Velcro-backed material, which easily affixes to the random orbital sander and angle polisher.
# 7a, 7b & 7c. 3M (Abrasive Systems Division) Finesse-it Plastic Repair Products. Shown are disc cutters for grinding and dimensioning (left), which use a Velcro attaching system. Polishing discs for fining and finishing (right) use an adhesive-backed attaching system. Polishing Film (center) is used for final buffing and clarity (also adhesive-backed).
# 8. A Random Orbital Sander with two types of 5" pads, which attach to the sanding and polishing discs using Velcro.
# 9. A high-volume air compressor is needed, capable of maintaining 120 psi continuously.
# 10. Good old 100-mph tape (duct) adheres well to Velcro, easily transforming any pad to accept adhesive-backed discs.
# 11a, 11b & 11c. This Cessna 150 windshield, although already 27-years old, responded well to restoration after mild hazing from nine years of outdoor storage and sand-blasting in the Southwest desert.
#12. Interior views, Cessna 150 windshield, although already 27-years old, responded well to restoration after mild hazing from nine years of outdoor storage and sand-blasting in the Southwest desert.
#13. A near-new view after restoring from a severely hazed rear window (Cessna 150).
#14. A big no-no. Never store anything on the panel where plastic windows are certain to scuff, scratch and wear. Carelessness can damage windows instantly.
Copyright (C) 2001 Ron Kilber firstname.lastname@example.org RonKilber.tripod.com Non-commercial reproduction permitted in its entirety with this copyright notice intact.