peter2407
Well-Known Member
I have heard of there being a mythical treatment for crazed perspex glazing. Any recommendations, success or horror stories?
Crazed perspex glazing - how to fix?
- Thread starter peter2407
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nimbusgb
Well-Known Member
there are polishes and processes available to polish out the crazing. Its quicker, more effective and possibly cheaper to replace flat panels of acrylic though
30boat
N/A
You can't really repair it only remove the crazed surface with pollish wich weakens it.
A
Anonymous
Guest
I've tried following the advice posted here and elswhere but nothing really works. It's better to replace the material and maybe not as expensive as you'd imagine.
saxonpirate
Well-Known Member
Yes, I'd agree with the above comments. It is possible to polish out the minutest of crazing/scratches with brasso or similar but any serious crazing, forget it.
VicS
Well-Known Member
As said surface scratches can be polished out with "Brasso" or a Perspex polish, crazing is the effect of UV light and can go several millimetres deep.
Replacement is the only option.
You now have the oppotunity to "upgrade" to polycarbonate if you wish.
Replacement is the only option.
You now have the oppotunity to "upgrade" to polycarbonate if you wish.
CPD
Well-Known Member
This is a "next season" job for me. Sorry for the ignorance, but why is polycarbonate considered an upgrade ?.
A
Anonymous
Guest
Polycarbonate is hugely strong -- but maybe you don't need the strength. It is also very prone to scratching. All you said was 'glazing'. If you give more details there is a good chance that someone here has done a similar job on the same or similar boat.
wiggy
Well-Known Member
If the crazing is on the surface but too deep for polish you can use we and dry and work your way to polish, in my experience most crazing is internal and polishing will give limited iomprovement
rob2
Well-Known Member
If you get round to replacing, a few observatins from our experience. Cut the shape with a jigsaw at slow speed with a metal cutting blade. Go too fast and the material melts! If the panels are unframed, the objective is to hold them onto a bedding of mastic, drill the screw holes a little oversize to allow the material to flex without stressing the panel. a slight countersink to the holes will also help to remove stress raisers, lthough the screw should be flat heads with washers to spread the load - put some mastic under the washer and into the hole to prevent leaks. Don't overtighten.
It's very satisfying to look through your new, clear portlights!
Rob.
It's very satisfying to look through your new, clear portlights!
Rob.
30boat
N/A
[/QUOTE] a slight countersink to the holes will also help to remove stress raisers, lthough the screw should be flat heads with washers to spread the load - put some mastic under the washer and into the hole to prevent leaks. Don't overtighten.
Rob.[/QUOTE]
Instead of countersinking wich gives you a conical shaped hole,it is better to use a flat bit to enlarge the hole for the (flat) screw head.
Rob.[/QUOTE]
Instead of countersinking wich gives you a conical shaped hole,it is better to use a flat bit to enlarge the hole for the (flat) screw head.
Ehbendisdonc
Well-Known Member
It may be worth pointing put here that polycarbonate, while very strong, is also particularly prone to crazing (environmental stress cracking). This is often as a result of exposure to chemicals such as adhesives, cleaning solvents or fuel. And as the previous posters have said, it's also important to limit the internal stresses.
I'm not sure whether perspex is better or worse in this regard.
I'm not sure whether perspex is better or worse in this regard.
VicS
Well-Known Member
When i suggested polycarbonate as an "upgrade" I deliberately put the word Upgrade in quotation marks. Its great merit is its high strength, almost unbreakable. It is the material riot shields are made of. As pointed out is does have some disadvantages and it is more expensive than Perspex or other brands of PMMA. It is particularly vulnerable to cracking due to solvents and for this reason care is needed in selecting suitable sealants.
Countersunk screws should not be used with plastic glazing as that can cause cracking to propagate from the holes. Also screw holes should be oversize to allow for expansion and contraction .
Countersunk screws should not be used with plastic glazing as that can cause cracking to propagate from the holes. Also screw holes should be oversize to allow for expansion and contraction .
srp
Well-Known Member
Machining acrylic (ICI trade name - perspex) should be done with a high tool speed and a low feed rate to avoid cracking and minimise the temperature stress cracking that can sometimes occur. Ideally the blade should be ground especially for acrylic, with a lower rake angle and no side rake, compared to the geometry of a wood cutting blade. In practice this is seldom done just to cut the odd few pieces. A bandsaw with a wood cutting blade will give a reasonable cut if the work is fed slowly, and the same applies to a portable jigsaw.
There are two sorts of acrylic sheet available - cast and extruded. The extruded sort is commonly found in DIY stores for glazing sheds etc. It is very prone to stress cracking and much more likely to melt at the point of cutting, so needs to be avoided. Signwriters usually use the cast variety, which is much easier to cut and drill.
Drilling close to the edge is usually when breakages occur - again, a high tool speed and a very low feed rate will avoid this happening. If you have a small grinder, take off the rake angle so the drill has more of a scraping action - a rake angle of 1-5⁰ should work well. Make sure the work is clamped to a backing block so it is fully supported when the drill breaks through. It may also be worth drilling the holes before the profile is cut out. I've never had any trouble with countersunk holes in thick pieces of acrylic (>10mm) but agree that on thinner sheet it is not a good idea - annoyingly, the cracking around the hole often appears a few days after fitting.
There are two sorts of acrylic sheet available - cast and extruded. The extruded sort is commonly found in DIY stores for glazing sheds etc. It is very prone to stress cracking and much more likely to melt at the point of cutting, so needs to be avoided. Signwriters usually use the cast variety, which is much easier to cut and drill.
Drilling close to the edge is usually when breakages occur - again, a high tool speed and a very low feed rate will avoid this happening. If you have a small grinder, take off the rake angle so the drill has more of a scraping action - a rake angle of 1-5⁰ should work well. Make sure the work is clamped to a backing block so it is fully supported when the drill breaks through. It may also be worth drilling the holes before the profile is cut out. I've never had any trouble with countersunk holes in thick pieces of acrylic (>10mm) but agree that on thinner sheet it is not a good idea - annoyingly, the cracking around the hole often appears a few days after fitting.
Elessar
Well-Known Member
Csail - the chap in Rhoose who supplies boat stickers - I think now has a product for this?
His name may have changed with the new forum and maybe I'm wrong - but try a PM to him if you can.
His name may have changed with the new forum and maybe I'm wrong - but try a PM to him if you can.
VicS
Well-Known Member
Csail is now known as "bonnym". His website is http://www.h2o-marine.co.uk/