Adhesive - Perspex Windows

[maxi]

My perspex windows, replaced two years ago by 'professionals' using Sikaflex or similar, have now completely detached, quite cleanly from the perspex side of the joint.

Question 1; is there a recommended preparation/cleaning procedure for perspex prior to sealing?

Question 2: Has anyone had experience of fitting windows using the double-sided-tape method, and what was the outcome.

Thanks in advance.

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[Talbot]

I have used a double sided tape with a bit of foam between the sides of the adhesive (made by 3M) This is supposed to be waterproof. Either the fibreglass was not sufficiently straight (a definite possibility) or the bolts were to far apart (another possibility), but the join is not 100% perfect. It is great for where I have used it (windows of the wheelhouse) cause the joint looks reat, and it is very neat), but would not use it on a cabin window.
Sikaflex make a specific glue for joining perspex to fibreglass, and I know of one multihull that has attached its windows purely with this sikaflex. However, this is not the normal stuff, so I suspect that your windows were installed with the wrong sticky. Have a word with sikaflex themselves and check which one they should have used. This is the way it should look:

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[tugboat]

Hi maxi. I purchased a boat this year with through bolted windows which are all leaking. The mastic has given way in places but a lot of the leakage is through the bolt holes themselves. The bolts are at 100mm centres and the surveyor recommended intermediates being drilled and fitted. Also that a polyurethane based mastic is used.

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[boatmike]

Common problem. 3 reasons it goes wrong. 1. the wrong grade of Sikaflex used. 2. Insufficient thickness. 3. Cleanliness. The correct grade is I think 296UV but check with Sika cos I can't remember the numbers. Point is it says UV after the number which means it is UV resistant. Ordinary Sikaflex aint. There is a Sika publication that will tell you exactly how to do it. Contact them directly and they will send you one. Even when the job is done it is prudent to cover the joint with UV protection tape (also Sika supply) Otherwise the sun will get to the bond in time and break it down even with UV resistant grade. Clean both the GRP and perspex thoroughly before bonding and then apply special primer (Sikaflex again) to both parts and leave for an hour before bonding. make some form of support for the bottom of the window to stop it sliding down while the sikaflex hardens. You will then need to apply sufficient sikaflex to give about a 5mm thick minimum layer when in place (spacers help) Thats assuming your windows are relatively small. If bigger, more thickness of Sikaflex. This is to facilitate the different rates of expansion and contraction of GRP and perspex. Again their publication gives you details. I prefer to have no bolts. They are unnecessary, cause leaks themselves, and stop the relative movement of the window and GRP when heated/cooled. If you MUST have bolts drill the holes out as big as possible and fit washers to allow movement. For those who poo-poo no bolts.....Look at your car windscreen. No bolts! Look more carefully and you will see a printed black edge to the windscreen to stop the sun getting at the seal..... Get the Sika booklet. Do EXACTLY what it tells you and you will have leak free windows. Guaranteed. Take short cuts and you wont.

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[snowleopard]

sikaflex 295uv

got the empty bank balance and leaky boat to prove it!

the stuff they sell for this job is sikaflex 295uv at anything up to £35 a tube. the preparation involves abrading both surfaces, cleaning with a special solvent then priming both surfaces using a clear primer for the grp and a matt black one for the acrylic. you then apply the mastic in a layer at least 3mm thick, more for larger windows. one tube covers around 1 metre x 50mm wide.

in our case the primer separated from the acrylic after 18 months. we then added bolts all round but the rain still poured in so we scrapped the lot and went over to neoprene foam strip.

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[gibbo26]

Try Neutral cure silicone rubber sealant/adheasive (Dow Corning is one manufacturer) it does not smell of vinegar as the standard type does(which is not recommended for perspex), has a longer working time (you can smooth off a joint a lot longer than the standard type) and its more resistant to uv than polyurethane. My forhatch perspex has been in place now for 10 years without any screws and with no sign of bond failure.

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[maxi]

Thanks Guys, some interesting points.

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[boatmike]

Re: sikaflex 295uv

What you didn't do though was protect the joint from UV by using protection tape. This is essential. Again you may have fitted bolts too tightly or not have had sufficient thickness (also essential). I have fitted glass, acrylic, and polycarbonate windows on ferries, superyachts and my own boat this way and none have ever leaked. Again I can only say this is precisely how all car windscreens are fitted. It does work if you follow Sika instructions to the letter. If you do it correctly, have a minimum thickness of 5mm of Sikaflex for even the smallest windows, and forget the bolts and use protection tape afterwards (not to stop leaks but to keep the UV out) the result is excellent.

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[maxi]

Re: gibbo26

Hi gibbo. I was interested in your solution & contacted Dow Corning, who came back to me with one of their technical whizz's, they are adamant that they have nothing in their range capable of attaching windows, only sealing a joint made by other means. Like you, I would have no qualms about re-setting my forehatch though.
Thanks anyway.

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[Abigail]

Hi - for our below deck level windows which are frameless perspex in a chamfered gap - we used:

1) interlock screws which work by pressure and are much safer and stronger than any screw (though expensive)
2) neoprene foam as a gasket, which is the key sealant
3) Dow& Corning silicon sealant around the outside - recommended by our window supplier.

After much misery related to getting the sizes of the windows wrong (a different problem!) these have worked very well. We have one small leak in one (of 10) windows (each with about 20 screws in) which we think is because we didn't quite fill one of the previous screwholes absolutely perfectly and it is squeaking at the edge of the neoprene. (one of the jobs on our list is sorting this small seepage out.)

This has withstood several nasty storms and two heavy winters with no problems at all.



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