Do I have to predrill before running screws into fiberglass - Page 2 - SailNet Community
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post #11 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

You will likely not be able to use the butyl along the length of the wood between the screws. I re-bedded the wood eyebrows on my cabin top last year and the screws are about 1.5' apart. Not enough force to compress the butyl properly. I used butyl around the screws and Sikaflex 291 under the wood.

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post #12 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

Better yet.... To improve the previous instructions...

Quote:
Drill the holes in the wood.
Put the wood over the fiberglass and tape it in place.
Put a bit in the drill that is the same size as the holes in the wood, so you're using the hole in the wood as a guide.

Drill through the wood and into the fiberglass.
Remove the wood.
Chamfer the hole in the fiberglass just a bit.
Replace the wood.
Install screw. (this will hold the wood in place and not allow it to move.
Drill the rest of the holes.
Remove the wood, chamfer and finish the job.


Slip the bit into the hole and just kiss the surface of the fiberglass.
Repeat for the other holes.
Remove the wood.
Put the proper sized bit in the drill.
Drill the pilot holes at the point where the drill kissed the fiberglass.
Chamfer the edges of the pilot holes.
Put the wood back over the newly drilled and chamfered holes, and add the screws.
While it might seem like a lot of extra work, I've seen (and suffered) the consequences of having stuff NOT QUITE LINE UP. And if you think it still might slip, do two holes before drilling the rest. The more holes you are making the more likely one won't line up properly. And we won't even talk about mating two surfaces that aren't flat and square to one another!
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post #13 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

As for the size of the hole in the fiberglass, making a hole larger than usual for wood has already been suggested. There seems to be no table for pilot holes in fiberglass.

An method that has worked for me was to acquire a scrap piece of fiberglass, start with the appropriate pilot hole for hardwood and drill and test increasing larger holes until the more or less optimum hole is discovered. That hole size seemed larger than I had anticipated.
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post #14 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

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Originally Posted by abrahamx View Post
Thanks. I'm glad I thought of that. Guess thats why they pay you the big bucks. And I'm a metal fabricator for 19 years. To used to measuring everything I guess.
Ask Chef2Sail and ChucklesR - I tend to be the one to overthink things.
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post #15 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

Which wood are you replacing?

As a general rule, horizontal deck surfaces are usually cored, and vertical surfaces are not. I'm sure there are many exceptions to this rule.

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post #16 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

If you drill through gelcoat - perhaps not in your case - start off by drilling in reverse. Reduces the likelihood of chipping.
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post #17 of 24 Old 08-01-2013
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Predrill all the holes first through the wood and the fiberglass. Then pull the wood off and then chamfer the fiberglass holes.

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post #18 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

Quote:
Originally Posted by paul323 View Post
If you drill through gelcoat - perhaps not in your case - start off by drilling in reverse. Reduces the likelihood of chipping.
This is a good tip. After drilling the small hole in the glass use a non-pilot bit whose size is about the size of your chamfer and - with your drill in reverse - start the chamfer by smoothing the edge of the hole before using the countersink/chamfer bit. If your large bit has the appropriate face angle you may be able to complete the chamfers with the bit eliminating the use of the chamfer bit (and higher potential for chipping the gel coat).

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post #19 of 24 Old 08-01-2013
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Re: Do I have to predrill before running screws into fiberglass

I'll add:

Using a large drill bit for a chamfer bit does work, but, doesn't leave a nice chamfer. Invest in a chamfer bit or use a router bit(in a drill).
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Re: Do I have to predrill before running screws into fiberglass

Thanks, I did not know there was such a thing.
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