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Rebedding skylights on Hunter 35

Oct 28, 2013
Hunter Legend 35 Fairfield, CT
Noticed a leak in both of the larger skylights on our 1987 Hunter 35. Took both of these skylights out and got as much as I could of the old sealant off, but that stuff has REALLY bonded to the fiberglass. Does anyone know what sealant was used for these windows? Tried our magic Butyl tape, and shockingly, it did not hold... so rethinking the materials. Any suggestions? Also, any ideas on how to remove it? Thanks a lot!
Jun 11, 2011
Hunter 41 Lewes
ArkadiyS, you need an adhesive in this case. The butyl is a sealant and perfect in scenarios where you apply constant pressure as in bolting and screwing. I think Sikaflex is an appropriate choice but ask Main Sail, he's the guru.
Last edited:
Feb 20, 2011
Island Packet 35 Tucson, AZ/San Carlos, MX
that stuff has REALLY bonded to the fiberglass.
Sounds silicone-like. Solvents aren't very effective, IMO. Sanding (and the like) produces heat which further impregnates your already silicontaminated top skin.
You may be able to physically scrape the fiberglass down to an un-polluted depth using a sharp card scraper, then re-lay some glass if necessary.
Hope others chime in.
Feb 2, 2006
Hunter Legend 35 Kingston
I have the same boat, and did a full replace of the 4 skylights:
  • Used Down 795. Has LOTS of stretch, and that's good because the skylight and hull expand and contract in the sunlight at different rates.
  • Had new skylights made to match the old shape but without the mounting holes the originals had
  • The hull opening needs to be very clean, and I roughed them up with sandpaper to ensure the 795 stuck well
  • I filled all the old screw holes that were used to hold the old skylights down (just to prevent any future leaks from getting into the hull/liner/etc.
  • I roughed up the edges of the new skylights so the 795 had a good surface to bond to.
  • Bonded them in only with 795, and no screws.
  • Used lots of tape to make nice clean lines
Important notes:
  • The cabin top is curved from front to aft
  • The skylights are not!
  • Simply pressing/weighting them in place would result in the sealant being squished out more in the center of the window (for and aft wise). Thin areas of sealant don't have enough material to stretch to accommodate expansion and would/could fail early.
  • When installing, I used small bits of plastic "tile spacers" to ensure that no part of the skylight was pressed down to the hull by too much and that the sealant had an even thickness. probably only used a 6 for each skylight, strategically placed.
  • I used lots of 795, and also used it to cover the entire "shelf" in the hull that holds the skylights, really just so I wouldn't see white gelcoat through the skylight.
  • I used ropes and straps laterally from toerail to toerail to hold the skylights in place (with blocks of wood as wedges) while the sealant sets (a few days for sure). Ratchet straps (low stretch) would be good, and preparation is key as the 795 does start to skin up fairly quickly and it's a big job to bed them, tidy up the joint, remove tape, etc.

The result was almost perfect. I have one corner that will form a drop of water after 1 or 2 day long rains. Upon inspecting, I can see no bond failure (but it must be there). I assume that the 796 didn't fully stick to some small portion of the skylight or the hull (grease? silicone leftovers?), and water will very slowely wick through the gap. So ... clean with solvents well!

Jan 4, 2006
Hunter 310 West Vancouver, B.C.
Check the archives and you will find a library of material there.
The sealant is Dow Corning 795.
Oct 28, 2013
Hunter Legend 35 Fairfield, CT
Thanks guys!! I am having a heck of a time getting the existing sealant off the fiberglass. Would DOW 795 bond to its 'older' cured version? I will sand it again, but not sure if all of the old sealant will come off. Also, I was wondering if it makes sense to use real screw fasteners that will go through the deck to hold the skylights in place, instead of just having them float on top of the sealant. Any suggestion?
Feb 2, 2006
Hunter Legend 35 Kingston
Personally, I had to tighten the screws that held my skylights down 2-4 times per season. As the boat flexed and/or the materials expanded and contracted differently, the screws kept working loose. Screws and screw holes are always a potential source of leaks too. The 795 is more than strong enough on it's own (if you can get the surfaces clean and free of old sealant).

It does not stick to itself well at all.

Jan 4, 2006
Hunter 310 West Vancouver, B.C.
Also, I was wondering if it makes sense to use real screw fasteners that will go through the deck to hold the skylights in place, instead of just having them float on top of the sealant.
In the Hunter factory, all of our fixed port and hatchlights were screwed in place to allow the 795 to set and then the screws were removed. The remaining small holes were then filled with the 795. Black on black and it's almost invisible. I know I have to look real hard to find the screw holes on mine.


As far as new 795 sticking to old 795, here's a note (below and lengthy) I received from Dow Corning a couple of years ago when I was replacing a small fixed port:

Dear Ralph,
This is a follow-up message from Dow Corning to your recent inquiry. Yes, 795 will adhere to old 795 as long as the old cured 795 is clean, dry, and frost free. Below I have provided information that discusses the removal and replacement of cured silicone sealant and our recommendations.
Removal and Replacement of Cured Silicone Sealant

A properly designed and installed silicone joint will typically last 20 years without need of replacement. In instances where the joint has experienced mechanical or other damage and replacement is required, follow the procedures below. Assess the problem with the joint.

1. If the sealant is cured properly and performing application but its appearance is poor (i.e., due to improper tooling), then cleaning the sealant surface with a solvent and recapping the joint should be sufficient.

a) Clean sealant with a solvent (i.e., xylene, toluene) to remove dirt. Allow solvent to evaporate.

b) Remask the joint.

c) Apply a thin bead of fresh sealant over the cured sealant.

d) Dry tool the sealant.

e) Remove the masking material.

2. If the sealant is mechanically damaged and a recapping will not improve the joint appearance, then remove the section of old sealant and replace it.

a) Cut away the old sealant. If excellent adhesion to the substrate is still maintained, then leave some sealant at the edges of the joint (up to 0.08"/2 mm thick).

b) If adhesion to the substrate is poor, then remove sealant down to the substrate and clean the substrate and recondition if necessary (i.e., clean with xylene and prime with appropriate primer).

c) Mask the joint.

d) Reapply the sealant. (If resealing does not occur on the same day, the joint will have to be recleaned using a solvent such as xylene or toluene before applying the fresh sealant.)

e) Dry tool the joint.

f) Remove the masking material.

g) Check adhesion after the sealant has cured for 7 days.

Thank you for contacting Dow Corning. If I can be of further assistance, feel free to contact me.
Best Regards,
Mary Altenburg
Technical Information Center
Dow Corning Corporation
We help you invent the future ™
Phone: (989) 496-6000 Fax: (989) 794-5900
Email: tech.info@dowcorning.com
NOTICE: (Please Read Carefully):
Dow Corning believes that any product use recommendation or product information presented is an accurate description of the typical characteristics or uses of the product, but it is your responsibility to thoroughly test the product in your specific application to determine its performance, efficacy, and safety under your conditions of use. Suggestions of uses should not be taken as inducements to infringe any patent. Unless Dow Corning provides you with a specific written warranty of fitness for a particular use, Dow Corning's sole warranty is that the product as supplied will meet Dow Corning's then current sales specifications. DOW CORNING SPECIFICALLY DISCLAIMS ANY OTHER EXPRESS OR IMPLIED WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND OF FITNESS FOR PARTICULAR PURPOSE. Your exclusive remedy and Dow Corning's sole liability for breach of warranty is limited to refund of the purchase price or replacement of any product shown to be other than as warranted, and Dow Corning expressly disclaims any liability for incidental and consequential damages.

Good News!! Many Dow Corning technical data sheets as well as Material Safety Data Sheets (MSDS) are now available online! Just visit our site: www.dowcorning.com

You may search under the product name or number, product family or industry. Some of this information is also available in several languages - just choose which country and language you want.

After receiving this and being a dyed in the wool skeptic, I placed a bead of new 795 on the some old 795 which was still on a lens from a Lewmar opening hatch which I had recently replaced. After a week, stuck to the old 795 like its life depended on it.
Oct 28, 2013
Hunter Legend 35 Fairfield, CT
Ralph, this is GREAT info! I did not find this when searching the web or in the forums! Any idea if this project is worth doing now (the boat is in CT) and for those who are familiar with weather and temps on the Long Island Sound), or should I just cover it and come back to it in the spring with milder temps? Thanks!
Jan 4, 2006
Hunter 310 West Vancouver, B.C.
Any idea if this project is worth doing now (the boat is in CT) and for those who are familiar with weather and temps on the Long Island Sound), or should I just cover it and come back to it in the spring with milder temps?
The one thing that always bugs me with these fixed glass projects is the great depth to which the 795 must cure. If you look at the width of the FG landing on which the glass rests, the cure must proceed from both the outside and the inside, so the effective depth of the 795 is 1/2 of the landing. The job I'm looking at has landings of about 2". No idea HOW LONG this take to cure, but I don't think it's just a few days.

So, I imagine you get pretty cold out Long Island way ................... will the 795 even cure at those low temperatures and stay fluid all winter long, or do you wait for spring when the warmer weather will speed up the curing action. I try to do all my inside work in the winter and outside work in the spring if that helps. Either way, I think the small screws should remain in for at least a few weeks. Check for thermal expansion of FG vs. acrylic and oversized holes in acrylic in the archives. You'll know it when you see it.

The funniest description I've ever seen for installing fixed ports/hatches was put out by Catalina some time ago. Look at paragraph g.

Catalina Instructions - Garbage.jpg

I got this from another site (the name of which escapes me) and it was followed by some poor guy cursing and swearing about loading his portlight with everything described in the instructions and then some. The acrylic kept sliding around. After he thought he had it set and left it for a few days, he removed the restraints and the glass opened up after a week or so. I'll stick with the temporary screws.

When I finally get fed up with the tiny, re-appearing leak in the big front fixed hatchlight (going on five years now), I intend to install a new one in the spring and leave the small screws in for most of the summer before removing.
Oct 28, 2013
Hunter Legend 35 Fairfield, CT
Someone has to 'pay' for this additional light in the cabin... having temps dip below freezing points at night, I will just cover it all up, waterproof it, and come back to finish in the spring. I will also experiment with a different butyl tape, and DOW 795. Thanks again!