Hatch window, sikaflex 291 deterioration

[AndersGu]

Hi! The hatch on my boat has a small window, definitely not Lewmar /forums/images/graemlins/wink.gif The hatch is molded in GRP, there is a cutout and over that there is a piece of plexi, heat formed to fit the curvature of the hatch. Underneath there is a teak combing made of 25x5mm teak ribs. It is sealed with sikaflex and bolted through.

I replaced the plexi more than 5 years ago because it was old and cracked, but last year it started leaking. I dismantled it today and found that while the teak was sticking to the grp like, you know what (had to chisel it off), the plexi was loose and futher, the Sikafex that once held it in place had changed to a fine powder.

I use Sika a lot to fasten stuff all over the boat and it generally lasts forever, what could have happened in this case? UV breakdown? As the Plexi is see-through.

If so, what should I use as sealant?

 

[pvb]

291 isn't very resistant to UV degradation; 295UV would be better stuff to use. Even then, if the face of the bond is exposed to sunlight it can still deteriorate over time - the solution is to shield it with special tape. You can find comprehensive details on glazing with Sika products in the Sikaflex Marine Handbook, which you can download here.

 

[AndersGu]

Thanks, I will print it out and read it.

 

[davidbains]

I visited Eagle Boat Windows at SIBS and they firmly told me to use Arbo Mast BR butyl based sealing compound for windows and hatches.

 

[AndersGu]

OK. Noted. Never seen it around here, but I have friends in the UK that can send me some if need be.

 

[boatmike]

Quite correct but the tape soon deteriorates and falls off itself. Mask around the area and spray with matt black aerosol paint /forums/images/graemlins/wink.gif

 

[pvb]

Don\'t believe everything you\'re told...

[ QUOTE ]
I visited Eagle Boat Windows at SIBS and they firmly told me to use Arbo Mast BR butyl based sealing compound for windows and hatches.

[/ QUOTE ]Arbomast BR isn't suitable for use where direct sunlight is a risk. The Arbomast BR datasheet specifically says "Not suited to situations exposed to direct sunlight. If used in positions exposed to sunlight the surface of the sealant may craze."

 

[AndersGu]

Re: Don\'t believe everything you\'re told...

"Quite correct but the tape soon deteriorates and falls off itself. Mask around the area and spray with matt black aerosol paint"

Or machine down the plexi to make space for an 1mm aluminum strip on top?

 

[boatmike]

Re: Don\'t believe everything you\'re told...

That would do it too if you want to go that far. For full information on bonding and sealing windows Sika produce a booklet. "bonded transparency for a clear view" If you follow their instructions to the letter you don't need to use screws at all.
Try to get a copy from your local Sika distributor or through www.sika-industry.com. Briefly there is a primer (206 or 215) that you should use on the GRP prior to bonding and on the window abrade surface on bonding area and prime with 209. Then apply 295UV to give a thickness of at least 5mm (don't bolt up with screws if used and squeeze it all out again!) Afterwards use any reasonable form of protection for the outside of the window to stop direct sunlight.
I did my windows this way when I built the boat 7 years ago and I would have to chisel them off if I wanted to remove them now.
For doubting thomas' who says you need screws or they will leak or fall off look at your car windscreen. Its held on in exactly that way and is printed with a UV protection strip on the outside.... Take a look!
Reading your post again it looks as if you might be bonding to aluminium or direct to teak. The primer for al is 210-T and for wood I don't know but degrease the wood if its teak before bonding as teak is naturally oily.

 

[AndersGu]

Re: Don\'t believe everything you\'re told...

I run a 1970 beetle and the windscreen is set in a rubber gasket /forums/images/graemlins/smile.gif. Thanks for the tips though. I think I will go with 295UV and then let it cure, before tightening the screws. Screws and holes are already there.. so. Then mask and paint the rim.

No, there is no aluminum. The plexiglass sits on top of the GRP molded hatch.

The positive effext of a thicker Sika layer would also be that breakdown would be easier noticed.

 

[Dipper]

Re: Don\'t believe everything you\'re told...

Unfortunately, one of the few uses that Sikaflex 291 is not suitable for is glazing which probably explains your problem.