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Old 12-12-2017, 02:36 PM   #1
SKS
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Enlarging fiberglass hull cutout

Hi all!
I am looking to tap the collective wisdom and experience of the group.
What is the best tools and technique to slightly elongate an existing fiberglass hull cutout?
I need to make a 23" x 4" vent cutout more like 23 1/2" x 4". I have zero experience with cutting fiberglass and want to do a clean cut without cracking gel coat or making a wavy line. The trick to 90 degree corners are just one of my " head scratchers".
Thank you in advance.
Steve
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Old 12-12-2017, 02:51 PM   #2
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Hi all!
I am looking to tap the collective wisdom and experience of the group.
What is the best tools and technique to slightly elongate an existing fiberglass hull cutout?
I need to make a 23" x 4" vent cutout more like 23 1/2" x 4". I have zero experience with cutting fiberglass and want to do a clean cut without cracking gel coat or making a wavy line. The trick to 90 degree corners are just one of my " head scratchers".
Thank you in advance.
Steve
A vibrating multi tool. You can buy one at northern tool or Harbour freight. Including required blades about $30. Be sure to be generous with the blue tape.
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Old 12-12-2017, 02:57 PM   #3
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What Gordon said
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Old 12-12-2017, 03:05 PM   #4
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Any jig saw when fitted with an abrasive grit blade does a fine job of trimming fiberglass.

A layer of masking tape to show the cut line will help with brittle gell coat cracking .
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Old 12-12-2017, 03:46 PM   #5
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+2 on what Gordon said.

also known as an oscillating saw or Fein tool I believe.
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Old 12-12-2017, 04:07 PM   #6
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90 degree corners are an invitation to future gelcoat cracking. You'll do much better drilling a 1/2" or larger hole in each corner and then cutting between the holes with your choice of cutting tools.

A router is a good cutting tool as long as you have a straightedge to guide the router. I would say though, that it's not a tool for a beginner and I would want to make my learning cuts on something far cheaper than a boat.
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Old 12-12-2017, 04:44 PM   #7
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I agree with aboutman. A 90 deg corner will crack. I have used holes as small as 1/4" to prevent cracks, but what ever you do, don't make a square corner.
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Old 12-12-2017, 10:23 PM   #8
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A squared out corner can be eased afterwards very effectively into a radius with a fine round file such as that used for chain saw sharpening. Just use a small diameter file.

Just be sure whatever is being installed will cover that radiused corner no matter which method is used.

Better to avoid but it can be taken care of as above. Just use easy strokes .
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Old 12-13-2017, 07:37 AM   #9
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Quote:
Originally Posted by FF View Post
Any jig saw when fitted with an abrasive grit blade does a fine job of trimming fiberglass.

A layer of masking tape to show the cut line will help with brittle gell coat cracking .
I agree 100% with FF. With this setup you have a large degree of control over the cut. An abrasive blade or an ultra-fine tooth blade with the blade angle setting raked back works too, provided the gelcoat isn't to thick.
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Old 12-13-2017, 08:29 AM   #10
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Originally Posted by aboatman View Post
90 degree corners are an invitation to future gelcoat cracking. You'll do much better drilling a 1/2" or larger hole in each corner and then cutting between the holes with your choice of cutting tools.

A router is a good cutting tool as long as you have a straightedge to guide the router. I would say though, that it's not a tool for a beginner and I would want to make my learning cuts on something far cheaper than a boat.
+1
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Old 12-13-2017, 09:05 AM   #11
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"A router is a good cutting tool as long as you have a straightedge to guide the router. I would say though, that it's not a tool for a beginner and I would want to make my learning cuts on something far cheaper than a boat.[/QUOTE]"

On the Mohs hardness scale, /steel and glass have the same hardness. Fiberglass will quickly dull and destroy a router bit. The abrasive blades for a sabre saw are usually diamond dust. They cut glass easily.
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Old 12-13-2017, 09:18 AM   #12
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Thank you everyone!
This is great stuff and I have much less concern now due to all the input.
I'm thinking now:
Pre-drilled corners with oscillating tool.
QUESTION
Looking at these online I see various cutters. Are we talking the blade that looks like a partial circular saw blade?
Thank you again.
Steve
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Old 12-13-2017, 09:35 AM   #13
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As FF said, a jig saw will work also, and probably give more control. If cutting from the outside (gelcoat side) get some abrasive blades, or fine tooth blades with reverse teeth, ie they cut on the downstroke, unlike normal blades that cut on the upstroke - this will help prevent chipping - available at most hardware stores and HD and Lowes.
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Old 12-13-2017, 11:18 AM   #14
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I've used all sorts of tools for the purpose. Tools which do not 'lift' the chips off, into your face, and raise chips off of adjacent surfaces are to be preferred. Many of the blades you might have at hand will dull quickly when cutting 'glass. But you can throw away cheap blades at hand more quickly and cheaply than the drive to Home Despot. That said, I will trash a holesaw blade when appropriate; much easier than trying to mill a round hole with something else.

I most often will use a router and a solid carbide 'end mill'. Mark the hole with blue painters' tape. Cut a rough hole with anything handy, including the router. 'Nibble' up to the line with the router; cut in the direction that does not want to dig into the wrong place. Clean up the hole, including the corners, with a file.

Just for fun, here are a couple pics. The white stuff is Home Despot's fiberglass sheet used for the window frames, glued on to Lexan windows and glued on to the frame, on a Little Free Library. The wood is Walnut on a 'switch thingy' and forms the switch cover for 'designer style' switches. The wood is Antique Heart Pine on the receptacles. All done with a router and the solid carbide end mill. Not very difficult at all, and I really don't have steady hands.
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Old 12-13-2017, 01:58 PM   #15
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Part of every job is the clean up at the end.

Living aboard a GRP boat , while under construction , dust was a no no, as it could itch for days .

The abrasive saw blade method makes the least dust.
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Old 12-13-2017, 03:26 PM   #16
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Great thoughts. Thanks.
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Old 12-13-2017, 04:03 PM   #17
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Those CHEAP grit grinding blades for metal work ok cutting fiberglass without chipping in a circular saw. And also diamond blades.
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Old 12-14-2017, 09:19 AM   #18
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I have used a carbide bit in a rotary tool like a dremel or roto.zip. they are somewhat like a small dia rasp and do not cause a side pull the way a spiral cutter does. Cutting free hand with these are pretty EZ jyst creates a lit of FRP dust to clean up.
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Old 12-14-2017, 09:25 AM   #19
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I have used a carbide bit in a rotary tool like a dremel or roto.zip. they are somewhat like a small dia rasp and do not cause a side pull the way a spiral cutter does. Cutting free hand with these are pretty EZ jyst creates a lit of FRP dust to clean up.
I have blue-taped a vacuum cleaner hose onto the Dremel such that it is right up to the grinding tip in these situations. Run the vacuum while cutting - a little harder to hold the "assembly", but a*LOT* less cleanup after.

Also, the router collar accessory (cheap) is very good in this application for helping control your hand(s).
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Old 12-14-2017, 09:26 AM   #20
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Dremel tool. I like that suggestion also. Especially since I already own one.
Thanks.
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