Fitting Windows

[timentide]

I have read that to get a good seal on refitting windows it is advisable to fit 2mm shimms all around the window whilst the sealant is going off. Then remove them and finally tighten up slightly on the fixings when you have removed the shims.
I have also read of someone's cure is to fit plastic washers on the fixings (be they bolts or screws) and tighten up on the window with the new sealant in place.
Both methods seem to get over the problem of losing the sealant if you just tighten down the window and squeeze some of the sealant out and not getting a good seal where this has happened.
Has anyone tried or recommends either of the methods and if so what do you use for 2mm shims?

 

[Robin2]

I fitted polycarbonate windows before I heard of either of these tips. I held the windows in with about 4 to 6 wood screws (into a plywood structure) and, when the silicone had set I drilled the holes in the wood (holes were drilled in the windows before fitting) and fitted the bolts. So far (5 months) it has been successful with only 1 leak (21 windows) where, I think, the bolt is a screw and is not tight enough.

 

[steffen]

Did it with my escape hatches on deck. I used pieces of a narrow alu strip (10 x 2 mm) bought in a DIY shop. Works a treat because you can re-tighten them later.
After the sealant has cured, remove the alu shimms and fill up the holes left by the shimms.
When all is set, give the bolts another half twist or so.

I have just ordered 6 new polycarb (Lexan) window panes for my Phantom 32 and will use the same trick. The panes go into recessed openings in the hull.

I would not recommend silicone as a sealant. I intend to use
Sikaflex 295UV because of its UV resistance.

When using polycarbonate, be aware of the expansion of the material; about 3mm per meter so leave space around the panes and drill any holes 0,5mm larger then required by the bolt.

Success

 

[vyv_cox]

Steffen,
If you use polyurethane or polysulphide with Lexan you will have to replace all of them within a year due to stress corrosion cracking. Marine silicone sealant is perfectly OK in UV, at least for five years in my experience.

Do a Google search on polycarbonate stress cracking - you will find hundreds of sites with the same message.
Vyv

 

[steffen]

Hi Vyv,
Thanks for the warning. I will check on Internet (GE Plastics offers a wealth of knowledge as they produce Lexan) and check with Sika.

 

[jfkal]

Maybe in the UK. But NOT in the med. it is gone within a year

 

[jfkal]

Mine are OK still after 15 years. No problems.

 

[jfkal]

Silicone may be fine in the UK but not in the Med. I use Sikaflex and no problems after 15 years

 

[Robin2]

Does sikaflex react chemically with polycarbonate?

 

[steffen]

I checked with both Sika and GE Plastics (they produce polycarb as Lexan).
GE says: check with your sealant manufacturer (see www.geplastics.com).
Sika says: Sikaflex 295UV does not affect Luxan. Other Sikaflex products may however. (see www.sika.com).

Happy sailings

 

[vyv_cox]

Yes, it does. Solvent induced stress-corrosion cracking is a well known problem with polycarbonates. Amounts of solvent needed are very small and one of the applications that first made the problem apparent was people attaching adhesive stickers onto motorcycle helmets.

There is a very technical discussion at http://www.c-medica.com/c-medica PDF/Chemical Resistance of PC.pdf which basically says that acetone, isopropanol, ethyl acetate and methylene chloride, plus plenty of others, will attack polycarbonates at low stress levels and attack them a lot more at high stress. The high stress test is only run for 1 minute, by which time cracks have developed. All of these are regularly used in common sealants.

Polycarbonate windows that are attached by bolting with a sealant mastic are at considerable risk of cracking within a very short period. In my own experience, 6 mm polycarbonate with a polysulphide sealant formed first cracks in about 1 month.

 

[vyv_cox]

Putting together the Sika information and the technical stuff I found, it would seem that the Sikaflex 295 UV is formulated with a non-polar hydrocarbon. The medical research uses hexane as a carrier, which I assume is inert to polycarbonate, so it could well be this. Useful to know for future reference - no doubt better than silicone.

Vyv

 

[jfkal]

I guess not