David Bowden's money-saving technique for restoring acrylic windows to as-new condition.
Most yachts have acrylic windows and hatches and over time they will drive you crazy. Although acrylic is a marvelous material, it develops minute cracks and lines which eventually grow until the whole window becomes opaque.
The types and reasons for crazing are complex and a search of the web will tell you more than I can but aggressive cleaners and solvents like benzene and ammonia are a NO NO. Plastic mesh sun screens, which are regularly used on windows and also stress will promote crazing.
The smooth outside surface develops tiny pits and very small imperfections which, with the addition of more chemicals and time, become deeper and more extensive. It seems that crazing from the inside is less common because of the cleaner environment.
Once windows start to craze your choice is simple –- replace the window or try to remove the craze. This is the story of my experiment to remove the craze from the 10mm thick front windows on my catamaran.
I replaced the front windows myself about seven years ago, which cost me more than $500 plus several days of preparation and finishing. Fitting windows without securing screws or bolts so that they do not leak will be the subject of another article. Colleagues have paid up to $1500 to replace large windows and hatches so the replacement cost is significant but remains an option should the experiment not succeed.
After replacing those windows I continued to use plastic mesh sun shades. At one stage I used some plastic pipe to hold the mesh off the windows when we were off the boat for an extended period. A few years later I started to see evidence of crazing but more significantly, a continuous line of heavier crazing right where those plastic pipes had sat! This convinced me of the need to avoid contact between plastic screens and acrylic windows.
I had heard stories about removing craze from acrylic but had never met anyone with first hand experience. A search of the various web sites and yachting forums was also less than illuminating. It takes a brave heart to take coarse grade wet and dry and grind away on your window in the hope of producing a craze-free optically-clear window. However, with the prospect of saving more than $500, I was game to give it a try.
Trials and failures
The challenge is to find out how much acrylic to remove to get below the crazed area and how to do this quickly without ruining the optical flatness of the panel. The next step (I call it “bringing it up”) involves the use of progressively finer grade sandpaper and Micro Mesh disks to bring the window back to an optically clear condition.
With 10 mm thick windows I felt I had enough thickness to remove up to 1mm or more of material without affecting its strength. My initial attempts were far too timid as I was using 120 grit on a random orbital as the first step. While I found I could restore the finish, the craze remained.
After another five attempts experimenting with different grits and tools, I concluded that I was still not aggressive enough as the craze remained but I had gained confidence that I could bring the panel back to a clear state. This situation was compounded by several days of rain and overcast conditions, which prevented the craze showing until the end of the bringing-up process was complete and the presence of craze was plainly evident. Damn. Start all over again to take off more material.
At last the sun shone and I was able to see the craze even when the window was completely opaque from the heavy sanding. I found that to bring a metre square of acrylic back to optical clarity from the 60/80 grit sanding stage, took 60-90 minutes. With this knowledge and confidence in my bringing-up process, my focus moved to the challenge of removing a lot of material as quickly as possible to get below the level of the crazing.
Start with coarse grit wet-and-dry and sand evenly with a flat sanding pad. I used Dura Block, which is flatter and better shaped for hand sanding and making maximum use of a quarter sheet of paper.
Sand aggressively, with plenty of pressure and use water to avoid clogging. Sand in two directions at 90 degrees to improve scratching effectiveness of the grit and change direction every five minutes or so.
When changing to the next grade of grit, use a new direction (90 or 45 degrees) to the last one so that you can see when all the previous scratch marks have been eliminated and replaced by the new marks. For any inspections use a squeegee and some clean water to clean the surface and with good sunlight you will be able to see if any previous grit marks remain. You should also be able to see if any craze remains; check from outside and inside at different viewing angles. For a one square metre panel, the first very coarse sanding may take up to an hour, depending on the severity of crazing. Do not change to the bringing-up process until all craze is removed. Failure to remove the craze or old scratch marks may result in these features remaining thus requiring you to repeat the process. If in any doubt, sand a bit longer to ensure all previous scratch marks are removed.
For my bringing-up process I used 80, 120, 240 and 400 wet and dry. I then moved to a random orbital sander and a spray bottle to keep the panel cool and lubricated. I used Micro Mesh pads (hook and loop) 1500, 2400 and finished with 3600. Note: These numbers do not correspond to wet-and-dry grits (see Micro Mesh web site for correlation table). The final step was polishing with a Micro-gloss liquid abrasive and buffing with a slow-speed lambs wool polisher.
Micro Mesh pads appeared to lose some of their cutting ability after one or two uses. To save time and effort use new paper/pads if you detect diminished cutting action. If you are not generating residue because the paper has lost its scratching ability, you are wasting your time. You can use old pads as a final cut before moving to the next finer grade.
As a result of this project I am confident that light and even moderate crazing can be successfully removed and acrylic windows and hatches brought back to optical clarity. My remaining challenge was to find some way of measuring the remaining thickness of the windows after this process.
Some minor degradation in viewing through distortion caused by uneven levels of removal is likely to occur but with care this is hardly noticeable.
I have been successful in removing all craze from a number of panels but there remain a couple of panels where I will have to remove more material as some minor crazing remains.
In future trials I may resort to 40-grit or even a belt sander as my first and most aggressive cut for heavy craze removal.
Instead of using plastic mesh screens that touch my acrylic windows I will use some suspension system. Every six months I will use the random orbital sander and 3600 Micro Mesh for a few minutes on each window followed by liquid polish and the lamb’s wool buffer in an attempt to remove surface imperfections which may be the start of the crazing process.
As the cost of removing moderate crazing appears to be no more than two to three hours’ work and $10-15 per sq m, I am ahead and with regular care these clear panels should last another decade at least.
• A good hand sanding block
• Water container for wet sanding
• Spray bottle when sanding with random orbital sander and hand sander
• Squeegee to remove waste material and water before inspection
• Sheets of 40, 60, 80, 120, 240, 400 grit wet and dry sandpaper
• 5 inch Random Orbital Sander
• Micro Mesh pads (5 inch) hook and loop – 1500, 2400, 3600
• Micro-gloss #5 final polish liquid
• Pure cotton flannel to apply and initially polish before buffing
• Low speed polisher
• Lambs Wool buffer pad
Useful Web sites
• Google and type in Acrylic polishing, Craze removal, Acrylic crazing etc
• Micro Mesh - www.micro-surface.com
• Fixseal adhesives for installing hatches and windows – www.fixtech.com.au
• Sandpaper and Micro Mesh supplies Manly Qld. – www.TheSandpaperMan.com.au