Using fastners and drilling in fiberglass

[Rumline]

When you use a wood screw through the fiberglass, do you drill a pilot hole? If so how big relative to the shaft of the screw?

 

I'm paranoid about cracking the gelcoat.

[Overland]

When you use a wood screw through the fiberglass, do you drill a pilot hole? If so how big relative to the shaft of the screw?

I’m paranoid about cracking the gelcoat.

 

Absolutely. I've learned to go maybe one bit size up from what I normally might use. I just hold the bit in front of the screw and pick the largest size that still allows me to see some thread beyond.

 

And I've found that you're more likely to crack the gel coat with the screw than the drill bit. Also, if you're drilling a larger hole and want to drill a pilot hole for the larger bit, then just drill enough to get a good centering divot in the gel coat. If you drill deeper with the pilot, then you're more likely to catch the edge of that hole with the larger bit and pop off a bit of gel coat.

 

I think it's inevitable to have some chipping or crumbling around the edge of the hole, since the gel coat is so brittle, especially the white. But I've yet to have a disaster, knock on wood. The only visible scars I've made were inside the closet like I mentioned above, and then inside the battery box, where I drilled out a large hole to run 4/0 cable, but when I went back to the hardware store to get a grommet for the hole, they were out and only had one size larger. Stupidity forced me to try and enlarge the hole and the drill skipped and gouged the fiberglass about a half inch over. Fortunately it was inside the box and I've learned not to do that.

[Townesw]

 

If you drill deeper with the pilot, then you’re more likely to catch the edge of that hole with the larger bit and pop off a bit of gel coat.

 

 

Would you be better off using a small burr in a Dremel to drill or enlarge holes? Would this reduce the tendency to chip the gel coat?

[Overland]

Worth trying. I've had good luck with a sanding disk on a Dremel but I haven't tried using a burr.

[DavidS]

When you use a wood screw through the fiberglass, do you drill a pilot hole? If so how big relative to the shaft of the screw?

I’m paranoid about cracking the gelcoat.

 

I did some research on the web before I was ready to drill through the fiberglass. It was recommended to put down masking tape and drill through that to prevent splintering of the gel-coat. It was also recommended to wear a mask to prevent inhaling fiberglass particles.

 

[Rumline]

Thanks for all the responses, but I'm not worried about drilling gelcoat...done that many times, using the tape method that DavidS mentioned. But whenever I've drilled fiberglass before, it was to install a through bolt. That was pretty straightforward and drama-free, because the threads of the machine screw / bolt never engaged the material. I've never used a wood screw in fiberglass.  I'm worried that when I drive it in, the fiberglass isn't going to give/compress the way wood does and then crack.

 

Overland, thanks for the sizing info.  I guess with such a large pilot hole you're not trying to squeeze the fiberglass nearly as much as I was thinking initially.

 

MontanaOliver, I'd never heard of "grinding a negative flat" on drill bits before.  Good to know, and thanks for sharing!

[Overland]

Really good info, guys. Sometimes I'll use tape and sometimes not. I haven't noticed any difference in the amount of chipping, so I mainly do it in situations where I might scratch the surrounding gel coat somehow. I'll have to try the friction drilling.

 

And yes, always a mask when drilling or sanding the fiberglass. Sometimes I worry about those guys at Oliver. Doesn't seem like a lot of dust control is happening there.

[Rumline]

Interesting about removing the core around the hole. Makes sense. I've used 3M 4200 in the hole and around the bolt to seal these areas before, like when I drilled through the deck of my old sailboat, but I confess I never pulled those bolts afterwards to check for water intrusion / deterioration of the wood core.

 

What is the recommended way to remove the core material? How much should you remove?

 

What about just impregnating the wood core around the hole with resin?