How Do I Successfully Drill Thru Fiberglass? - SailNet Community

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Old 02-28-2011
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How Do I Successfully Drill Thru Fiberglass?

I need to drill a few holes thru my lazarette sides to attach a piece of hardware. The last time I drilled holes in this area was when I installed some hasps to be able to lock the lazarette down. When I drilled the holes that time there was some chipping of the fiberglass around the hole entries. How can I prevent this from happening the next time?

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Old 02-28-2011
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the most sure fire wire way to stop the chipping is to use a spade bit to start thru the gelcoat, the ones with the tips on the out side. then use a normal bit to finish, the problem lies with the spade bits dont last long. ( speed bore IIRC )

the other option is to try it in reverse until you are thru the gelcoat.

some people also drill it out in several steps, start small and drill out bigger, and bigger until it is the size you need.

or go buy some brad point bits, and use those but they wont last long either
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Old 02-28-2011
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I used a brad-point recently to mount my new Tacktick head unit, went through perfectly. Don't press on the bit, allow the bit to cut with as little pressure as possible. You might also want to use a bit of tape, I didn't bother as the brad point tends to prevent slippy-slidey anyway.
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Old 02-28-2011
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I agree with Scottyt about drilling it out in several steps, start small and drill out bigger, and bigger until it is the size you need. But I don't agree with the spade bits. Unless the spade bit is kept sharpened it will destroy the gelcoat much like the drill bit did to you before.

PaulinVictoria have it right as well with the brad-point. The tip regarding the application of very little pressure to the bit should be applied to all bits when being applied to fiberglass.

All the best.
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Another way.. 3 different drill sizes but a different order.

First use a smaller drill bit to make a hole.
Then use a bigger than needed drill bit to just drill through the gelcoat, (not the fiberglass) this will allow you to use the final drill bit, to drill through without causing the gelcoat to crack.

Practice first.
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Old 03-01-2011
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One more trick is to press hard or use thin two sided tape, on a sacrificial 1/4" sheet plastic or fiberglass against the surface. Then drill thru them both. It keeps the drill from grabbing the gelcoat and chipping it away. With tape you'll want to twist it off not pull off the plate. For alignment a small hole in the sacrificial plate is used to line up a mark (X).
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Old 03-01-2011
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Or you resharpen the bits used for fiberglass just as one would for plxie glass / steel / plastic / all mediums require different angles for the cutting face of any machine tool ..

Your answer is eazy to find on the web looking under machining.

Good luck.
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Old 03-01-2011
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The BEST way to drill into fiberglass (gelcoat) to prevent chips and tearouts of the gelcoat ... use a portable drill press (or first drill a very small pilot hole if you dont have a press), set up the alignment of the bore, put down a section of masking tape onto the gelcoat, and before your drill put down a sheet of fine (100 grit) sandpaper (face side down) on top of that, and SLOWLY increase pressure of the drill tip as you begin to penetrate through the inverted sand paper and masking tape. Will be 'perfect' hole and without 'tearouts' and flakes .... because the drilling begins as a 'grinding' due to the sandpaper grit ... the masking tape 'strengthens' the gel. Slowly PEEL the masking tape away (alsmost folding the tape over onto itself) when done, dont 'rip it off' nor pull 'up'.

Last edited by RichH; 03-01-2011 at 10:33 AM.
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Old 03-01-2011
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Bradpoint bits but start in reverse and then switch to forward once through. I've drilled literally thousands of holes this way with zero chipping..

You'll want a good set though. Many bradpoint sets are meant for wood only and are soft steel. You want a good high speed steel set. Rockler has a decent set and is what I am currently using.

Don't forget to countersink the hole after drilling to create a good seal for your sealant of choice if bedding the hasps.

You can see the holes they make in this article:
Potting Deck Penetrations To Prevent Core Rot

HSS Brad Point:


Clean Holes:


Done right and started in reverse a good set will even make near perfect holes in PAPER!




Starting in reverse is also a key trick with hole saws too. You can drill clean holes even though headliner fabric & vinyl by beginning in reverse. Once through the gelcoat, which is easy, flip it to forward..
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Last edited by Maine Sail; 03-30-2012 at 10:35 PM.
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Old 03-01-2011
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Yes MainSail , a Fostner type looks like the type to use for FG ...
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