Screwing into Fiberglass

[Cotemar]

Screwing into Fiberglass

We see it all the time.
Cracks around screw holes
Screws pulling out
Screw heads sheared off and the threaded part still in the fiberglass.

Here is a few tips.

Never use wood screws in fiberglass

1) Use 316 Stainless Steel Sheet metal Self-Tapping Screws

2) Review a drill chart to select the proper drill for the screw size you’ll be using.

3) Chamfer each hole to remove the gel-coat, so it will not crack.

4) Apply a silicon the seal and lubricate the screw going into the hole.

5) Only use a hand screwdriver to feel when the screw bottoms out.
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[Haida59]

Good information. Using a countersink to chamfer through the gelcoat until you hit fiberglass with take the load off the edge, lessening the chance of spider cracking. It also leaves a relief for more sealant allowing for more elasticity in the seal. Sikaflex 291 or 3M 4200 are a better choice for bedding to gelcoat and fiberglass than Silicone.

[Glenn.Brooks]

I typically use Dolphinite bedding compound under screws and bolts, even window frames on fiberglass boats. Yep, definitely old school. It was originally made for bedding wood and preventing dry rot. BUt it works magnificently to seal screwed and bolted fittings over FG and really any other boat building material. Also The fungicide in the compound actively resists green slim and mold growing around the outside of the base of fittings etc. and the material lasts 30 years- unlike 4200, which I've found has a maddening tendency to move and break it's seal against water over time- then be very difficult to dig out and replace. So, it's Dolphinite for me. BTW, I used Dolphinite exclusively to seal ever bolt, window, and fitting on 65 YO Alberg 30 refit. The new owner now has 10,000 NM cruising from the west coast to Hong Kong, via Hawaii, two trips to the So. Pacific, and The Phillipines - 30 months enroute. He recently emailed me to say he has NO leaks, anywhere on the boat. Very happy about that!

Glenn

[minaret]

I generally use a tap as well in glass. No cracks ever that way.

[roverhi]

Sillycone might work in boobs but only as a last resort in a boat.

Dolphinite is definitely old school like Nat Herreshoff ancient. Dolphinite will very slowly dry out and harden. The fungicidal Dolphinite will leach into wood and stain it. Use the regular stuff on bright wood.

If you want forever flexibility, use butyl.

[Cotemar]

We only use Boatlife Lifeseal on screws to lubricate and seal.
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[Cotemar]

Originally Posted by minaret

I generally use a tap as well in glass. No cracks ever that way.

We use taps for when be bolt through with a washer and nut on the back side or with larger size machine threads.

Tapping fiberglass actually works extremely well.

I have never seen a Sheet Metal Screw Tap, only Machine Screw Taps.
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[clymbon]

All good information, except for the suggestion to use silicone to bed the fitting. If the seal ever fails (and it will, eventually) it will be very difficult to rebed the fitting once silicone has been used. It leaves a chemical residue on the surface that is very difficult to remove, and nothing will properly bond to it - not even more silicone.

Whatever you do, DON'T use 3M5200. It provides a very strong bond, so use it to stick things together that you never intend to take apart. But, again, EVENTUALLY the seal will fail, and you will need to rebed it. The problem with 5200 is that eventually the SEAL WILL FAIL BUT THE BOND WILL NOT. Trust me on this - a previous owner of my (very old) boat used 5200 (or something like it) on some fittings. In some cases the seal has failed, and the fitting needs to be rebedded. In other cases, I just want to remove the fitting and seal the hole because that fitting is no longer used. I was eventually able to get those fittings off - but the bond between the 5200 and the underlying gel-coat was much stronger than the gel-coat, so now I'm faced with repairing a big gouge, rather than just filling a small hole.

Sikaflex is recommended. Also I've read good things about butyl tape, though I've never used it.

[danielamartindm]

Originally Posted by clymbon

All good information, except for the suggestion to use silicone to bed the fitting. If the seal ever fails (and it will, eventually) it will be very difficult to rebed the fitting once silicone has been used. It leaves a chemical residue on the surface that is very difficult to remove, and nothing will properly bond to it - not even more silicone.

Whatever you do, DON'T use 3M5200. It provides a very strong bond, so use it to stick things together that you never intend to take apart. But, again, EVENTUALLY the seal will fail, and you will need to rebed it. The problem with 5200 is that eventually the SEAL WILL FAIL BUT THE BOND WILL NOT. Trust me on this - a previous owner of my (very old) boat used 5200 (or something like it) on some fittings. In some cases the seal has failed, and the fitting needs to be rebedded. In other cases, I just want to remove the fitting and seal the hole because that fitting is no longer used. I was eventually able to get those fittings off - but the bond between the 5200 and the underlying gel-coat was much stronger than the gel-coat, so now I'm faced with repairing a big gouge, rather than just filling a small hole.

Sikaflex is recommended. Also I've read good things about butyl tape, though I've never used it.

I recently had to replace hatch COVERS instead of hatch seals because the original hatches were screwed together with 5200, and I couldn't back out the screws. Given that no construction or repair is forever, using 5200 doesn't make sense to me. Somewhere in our engineering, design, and repairs, we need to acknowledge that nothing lasts forever, and we should be factoring ease of replacement into what we're doing.

[Cheechako]

Originally Posted by danielamartindm

I recently had to replace hatch COVERS instead of hatch seals because the original hatches were screwed together with 5200, and I couldn't back out the screws. Given that no construction or repair is forever, using 5200 doesn't make sense to me. Somewhere in our engineering, design, and repairs, we need to acknowledge that nothing lasts forever, and we should be factoring ease of replacement into what we're doing.

I think you will find whether the screw will back out or not has nothing to do with 5200. It's just not that tenacious holding on metal and dealing with that twisting torque. I have probably removed hundreds of screws that were bedded in 5200 with no issues. It just comes right apart. Now the things 5200 is holding together are a different animal altogether!... except metal.

Regarding screws in glass, if there is no nut/washer on the backside I always tap and use machine screws. Fiberglass taps nice. If you are doing a bunch of holes use a tap in a battery drill. Goes right in and out fast.

[Colin A]

Originally Posted by roverhi

Sillycone might work in boobs but only as a last resort in a boat.

Dolphinite is definitely old school like Nat Herreshoff ancient. Dolphinite will very slowly dry out and harden. The fungicidal Dolphinite will leach into wood and stain it. Use the regular stuff on bright wood.

If you want forever flexibility, use butyl.

Silicone has it's place even on boats certain types of plastic for instance seal better with silicone then polysulfide. But for most work 4200 or boatlife are the go too's. I use Dow 732 silicone for certain things like some plastic port lights etc.

[Colin A]

If the glass is thick enough drill and tap is a good option. Of course if it's not that thick I would be careful mounting much without thru bolting.