Techniques to rebed dead lights (windows)

[hellosailor]

I guess the question is whether one wants to "form a gasket" or one wants to BOND the fitting in, as well as sealing it.

I'd rather bond, but I guess that's what makes horse racing.

[Sunspot Baby]

I want both, bond and gasket. I think I can have my cake and eat it too on this issue with a little care and attention to process and workmanship.

I also agree there is probably a fair bit of tolerance (slop) in the Sika recommended procedure. However, having also spent a fair amount of a previous life in technical pursuit, I have some respect for the "recommended" procedure which undoubtedly aims as the nominal or low risk result.

I don't want be anal, but by the same token, do I want to carelessly ignore recommended methods.

In my experience engineers are willing to take risks based on the best data available; lawyers who are not. I think I have been talking to engineers at Sika, not their corporate counsel. I am gaining respect for them (Sika) in our conversations. They are not careless, but seem well established technically. That doesn’t mean I understand of endorse everything they say; only that I find them believable.

George

[exposure]

I guess the question is whether one wants to "form a gasket" or one wants to BOND the fitting in, as well as sealing it.

With the Sikaflex you really do get the best of both. You get thickness and flex for sealing and a tenacious bond for strength. If I did not already have screw holes I would not even have bothered with screws to mount my windows.

[Sunspot Baby]

Between house guests and weather, I delayed starting the deadlight rebedding project until yesterday. Today I am sadder but not much wiser.

I started with a window, that should it leak would be the least problem. Figured my first would be the worst on the learning curve.

Well speaking of curve, it is in the head and is curved around the corner of the deck house.



I took the screws out no problem and figured the window would come free with a little pressure from the inside. Not so. This thing appears to have been installed with thick, black, somewhat flexible double sided tape. It is REALLY on there.

The window is so brittle, it cracks easily. Now, I am faced with two problems.

• Getting it off intact enough to duplicate.
• Finding someone who can heat mold a new one.

There is a window company in town that does acrylic and Lexan and maybe they can refer me to someone.

If anyone knows of a shop that does this work within a couple of hundred miles of New Bern, NC please give me contact information.

II had a lot of innovative ideas about getting the pane off, none of which worked. Will try heat gun next.

George

[hellosailor]

George-
If you go to an auto body supplies shop, or an auto glass shop, they can help you out on this. Automobile windshields are usually GLUED in place these days, so they have the exact same situation trying to replace them. The trick is a think strong wire, similar to a guitar string or piano wire. You can probably use "spider wire", the ultrastrong synthetic fishing line, instead. You make a small hole, using a knitting needle, drill, or heated canvas needle, etc., past the edge of the windshield and pass the wire through.
Then you tie each end, one inside one outside, to a bit of dowel. Now you've got a wire garotte. With a helper, one of you inside and the other outside, you saw the wire through the adhesive, around the windshield, and it will all cut free. Then the you the usual solvents & brute force to remove the adhesive--again, the auto glazing trade should be able to help you out on that. They often use a urethane bedding compound and yours may be a more conventional tape, but the same methods will work.
A heat gun, to soften the adhesive, may also help.

If you ask the local company about making replacement plastic--ask them about UV and scratch-resistant grades, often what they sell from stock is not "architectural" quality and the good stuff has to be special ordered at stiffer prices.

[Sunspot Baby]

One of the "innovative" techniques I tried was a piece of steel fishing leader. Wife inside, me out. I made some headway with it but it was really tough and even then, when I was working near screw holes with cracks, the cracks propagated like crazy.

I also slid a hack saw blade between boat and window. It just gummed up and wouldn’t cut.

I didn’t want to get too wild with solvents but used some turpentine. It helped a little, but I quit before I broke the glass up any more.

I’m headed to town to buy a heat gun. Should have had one anyway.

George

[exposure]

If the window is not recessed you might consider using a "sawzall" reciprocating saw. They make some very long and thin blades that are very flexible. Try using masking tape to protect the finish and use it from the outside. The hardest part may be to keep the saw steady so the blade will move and not the "user". IF the blade gums up, keep a couple handy and have your wife clean one with solvent while you are using another and swap them frequently. If the tape is not enough to protect the finish then use a piece of sheet metal taped against the boat with duct tape. I guess I was lucky and my windows were only bedded with silicon.

As for bending lexan, that is something that home aircraft builders do occasionally by simply using a form and an oven, so It can be done fairly simply. But, your pieces look a little big for a standard oven.

Best of luck, Woody

[exposure]

Also, If you haven't already, make a paper template now so at least you will have a good template for making the flat piece before forming. Tape some Kraft paper to the outside of the window and take a rubbing using a crayon or soft pencil. You should be able to get the hole locations as well as the perimeter shape.

[Sunspot Baby]

Exposure:

I was figuring on making a template before it breaks up any more. I think the sawsall may be a little agressive and any misalignment could chew up the window, figerglass, etc. I would be very nervous.

George

[Benjamaphone]

Originally Posted by hellosailor

I guess the question is whether one wants to "form a gasket" or one wants to BOND the fitting in, as well as sealing it.

I'd rather bond, but I guess that's what makes horse racing.

This is a very important point. The problem with Lexan is that nothing will want to bond with it for very long. As has been noted, screwing it down breaks any seal that had been made and kind of makes the whole proceedure a waste of time. On top of that, each screw hole, being 50% larger than the screw to account for thermal exapansion, is a recipe for leaks. Thermal exapansion works both side to side and up and down, gradually lifting and moving around your screws and holes. As soon as this happens, the seal is broken and you may as well have drilled holes in the top of the hatch.

Because of this, Lexan should not be used. I might sound like a broken record now with the whole Lexan vs. cast acrylic thing, but its surprising to hear that sailors are cutting corners like this. With cast acrylic, a felixible adhesive will permanently stick to the glass and the substrate. This will allow for thermal exapansion while maintaining a water tight seal for decades to come.

[hellosailor]

Ben, I know that A&H makes good stuff. But GE also has sold a lot of Lexan for exterior glazing, and they provide free installation drawings showing how it can be mounted properly, so that there are no issues with caulks, sealants, leaks, or expansion. It is a matter of gong beyond the obvious (i.e. screwing and gluing" and following the manufacturer's specifications for installation.

On expansion problems--every plastic has them. Some more than others, but they are all "alive" and even cast acrrylic needs to be able to move, or it will craze from stresses.

Out of curiousity, because there are many ways to estimate impact loads, what do you figure to be a sufficient thickness for cast acrylic, say for a 22"-24" wide deck hatch? Assuming that it may either be struck by falling rigging (i.e. spinnaker pole coming off a mast mount) or impacted by a breaking 40' wave of green water dropping onto the boat.

Or, do you figure the thickness to lesser impact loads?

Seeing that Lexan and other polycarbonates are normally 10x-100x stronger than acrylics, I'm curious to see how you figure that cast acrylic can compete with Lexan in that aspect.

A couple of years ago I was helping some friends replace some portlights (not A&H) and we quickly figured out they had been glued in and would not simply come out after unscrewing the trim. I said "Bust 'em out" and my friend said no, you can't just do that, they're too strong. Well, one "dink" with the hammer proved how unsafe typical portlights are. We busted 'em out, collapsed the trim, had no problems. But if they had been polycarbonate--busting them out would have required a much bigger hammer. MUCH.

Makes me prejudiced in favor of lexan, as you might have noticed.[g]

[Benjamaphone]

Yes all plastics have expansion problems. The difference is that cast Acrylite doesn't need to be screwed down to create a proper seal. When people screw their Lexan down, they are essentially trying to hold back the tide. The Lexan will expand/contract and those screws will shift around and crack any seal. By using a product like Acrylite that flexible silicone will actually stick to, we prevent this crack by letting the lens 'float' in a kind of way.

As for thicknesses, in an area where you would expect heavy traffic or activity, we would recommend a 1/2" lens. That is not standard and is available as an option. For most purposes we use 3/8" or even 1/4" on smaller hatches/ports in areas with less activity.

That 10x-100x stronger figure is very misleading. Its like saying the harder the steel the better. Hard/strong also = brittle. In applications like hatches where there is going to be a lot of flex and wear and tear, you want some give.

If the portlights that you were working on had to have screws to hold down the lens than it was probably Lexan. Cast acrylic does not need screws because silicone forms a very good adhesion with it.

As far as prejudice, in business you can't afford that. You use the best available products by determining what a specific application requires and you move forward. In our experience, cast acrylic lasts longer, leaks less and mainatins its aesthetics far better than Lexan.

[hellosailor]

Interesting that you prefer a sealant that will adhere to the glazing, as opposed to a floating seal. Of course putting screws through Lexan and other glazing (even with oversize holes etc.) is still, at best, a kludge job and not the real professional installation technique that the makers all suggest.

I think it was on this forum that one of the members mentioned a relatively new 3M product...something like "VLB" tape, a structural adhesive mounting tape (like a two-sided butyl mounting tape) that is actually used to attach glazing in skyscrapers and other places where most folks would say "tape" is simply asking for trouble. I have no idea how or if that works for polycarbonates, short or long term.

If, as you mention in the other thread, Acrylite is so vastly superior in UV resistance, that's enough to make it "better" for those of us who would prefer not to replace glazing every two or three years. I'd swear I've seen Lexan last longer than that though.

The portlights I was working on were very definitely not Lexan, they were some type of acrylic or cheaper plastic. No UV problem, just old age. Not screwed through, but set in plastic frames which were glued in the hull with silicone (by the maker) and then finished with trim rings that were screwed into the hull--giving the outer appearance that the trim rings were compression rings, not just cosmetics. Sadly they were just cosmetic and of course, the silicone eventually lost enough adhesion in enough places to make leaks between the frames (not directly on the glazing) and the hull.

[later]

I can see I'm going to be confused for a while longer. Acrylite is apparently a "Degussa" trademark in the Americas--but the identical material is sold as Plexiglass overseas. Yet Plexiglass is a mark owned byu someone else, successors to Rohm & Haas. Either way, Acrylite apparently is simply Plexiglass from another vendor, and in my distant past experience, conventional consumer silicones just didn't stick to plexi. I know technology has gotten better since then, among other things the wheel and fire have been invented.[g]

You're using something a bit more advanced, with a special primer, to get that bond?

GE claims that "Plaskolite GE57 Silicone Sealant" will bond to Lexan, perhaps that's also something new?

[GordMay]

Both Acrylic and Polycarbonate are thermoplastics, which means the materials can be re-melted from solid state, for use in injection or casting process.

Acrylic (Plexiglas, Perspex, Plazcryl, Acrylite, Acrylplast, Altuglas, & Lucite) is derived from methyl methacrylate. This plastic has excellent optical qualities, good chemical resistance and thermal and electrical properties, and has moderate strength. This strength can be enhanced by use of an additive in the formative chemical reaction, yielding high impact acrylic. The additive, however, reduces clarity, weatherability, and flexural modulus.

Polycarbonate (Lexan, Merlon, Makrolon, & Panlite) results from the linking of dihydric or polyhedric phenols through carbonate groups. The material has very high impact resistance, is easily processed by all thermoplastic methods, and has high temperature performance. Additives will make it resistant to ultraviolet radiation, which can cause long term degradation of the plastic.

[Whimsical]

Originally Posted by hellosailor

I think it was on this forum that one of the members mentioned a relatively new 3M product...something like "VLB" tape, a structural adhesive mounting tape (like a two-sided butyl mounting tape) that is actually used to attach glazing in skyscrapers and other places where most folks would say "tape" is simply asking for trouble. I have no idea how or if that works for polycarbonates, short or long term.

VHB or very high bond
Not realy new I was using it 20 years ago for mounting signage.
Funny stuff, it is an acryic adhesive that is not very sticky but after it has cured it is very tenacious and strong. It is used in many industries, the busses here have all the sheet metal held on with it.

Mike