Crazed Plexiglass - What to do?

[Barnacle Bob]

I have a large teak hatch with a heavy white translucent plexiglass top that is crazed or has what appears to be square cracks in the surface. It is possible to polish these out or heat them with a torch to smooth them back to the original finish? Or should I just resign to leaving it like it is?

[Scott Rosen]

Are you sure it's crazing? Maybe it's just scratches. Crazing comes from degradation of the plasitic, most likely the result of sunlight/UV damage. If the plastic has that kind of damage you should replace it with something more durable like Lexan.

If it's just scratches, there are some products you can buy that "restore" plexiglass. Most of them start with an aggressive compound to buff out the scratches and then move on to finer polishing materials. Other products simply fill the scratches or cracks and hide them. When the stuff wears off, all of the scratches come back.

[Ian McColgin]

Modern pharmacopias have a variety of remedies for the crazed - thorozine, ritilen, etc.

But seriously, sometimes a fine abrasive toothpaste like Crest on a fine cloth will polish it out if it's just the crazing that happens with age and not scratching from active wear.

[Alan D. Hyde]

Jeweler's rouge and a buffing wheel may do it, depending on the type of cracks you're dealing with.

Alan

[Barnacle Bob]

I got it right here in my kitchen and just took a look at it. It is a half inch thick. The inside is smooth, the exposed surface has some scratches (lines) but the rest is a checked pattern. It almost looks like you could take a blow torch and melt the surface back to it's original smoothness.

[Dave R]

There are polishing kits available that do a nice job. I've used them to polish plexiglass airplane windshields and had great success. One source is http://www.myles-rec.com/page27/page27.html The product is listed under "Best Scratch Removal System" almost to the bottom on the left side.

[Concordia..41]

The best product I've used for Lexan/plexiglass scratches, overspray, etc. is a system called Novus. It's a three party system of varying degrees of coarseness specifically designed for the purpose. Probably $20 for all three.

But two things to keep in mind.

1) If it's deeply crazed (now aren't we all), it's not going to get the really deep lines. You're just effectively buffing the surface.

2) Once a hatch has aged to the point of crazing, even if you get good results with the Novus now, you can expect the crazing to return pretty quick. The Chemist can chime in and explain how the UV damage impacts the properties of the plastics.

Happy polishing smile.gif

- M

[Bruce Hooke]

'Soft Scrub', available at your local supermarket, has given me very good results on crazed plexiglass, as well as being great for lots of other cleaning purposes on boats...

[paladin]

Replace with equal thickness of Lexan...cover when not using it.....should last 10 years or more.....

[Figment]

Bob, I caught this one over on that other forum.

I helped a friend reglaze the three hatches on his egg harbor over the winter. He'd decided to go from the charcoal-tinted to translucent white. Translucent white plastic (yes, that's really all you're dealing with) can be gotten pretty easily and cheaply at any industrial plastics house. The glass guys still haven't caught on, around here anyway.

[htom]

Plexiglass in a hatch cover is structural, as well as pretty. It will be much easier and cheaper to replace it before it fails.

[thechemist]

What paladinsfo and htom said.

[Adam C]

What Chemist, Htom, and others said.

Really, burning it 'back into shape' sounds like a recipe for disaster.

Get a new piece. You'll thank us for it.

[capt jake]

I've had pretty fair luck with 'flaming' pley and Lexan with a torch. It is touchy stuff though. Get your self a scrap piece to practice on. Once you get hte technique, it is a breeze. If you flame the surface, you will probably have to buff it out, now there is another touchy situation. Practice on some of that scrap to get the wheel speed and amound of compound just right. When you do it will gleam!!! :D

[thechemist]

*sigh*

That is called fire-polishing.

It leaves residual stresses in the plexiglass, only far worse than polishing.

Sunlight, or even time-in-storage will cause residual stresses in acrylic plastic to microscopically crack the plastic, which is "crazing".

Fire-polished acrylic will therefore craze even faster than ordinary store-bought sheet.

Residual stresses must be relieved after every polishing or machining operation, ideally, to avoid crazing.

Special machining requirements include drilling with cold soapy water for a cutting fluid, and grinding tools or drills for a cutting angle essentially ninety degrees to the work.

A required annealing schedule is 165 F soak of one hour for every quarter-inch of thickness, then cool to below 120F at a rate of no more than five degrees F per hour.

That relieves residual stresses in acrylic.

Lexan takes different treatments to anneal. I am not so familiar with its requirements, but I am sure there are some.

[capt jake]

Man ol' chemist!! Blow me out of the water! OK, good stuff to know. The fire polihsing that I have done has never been exposed to UV or sunlight (so to speak). It is fast and slick though! :D