How to cut fiberglass in a straight line

[jbinbi]

I would like to make a new cabinet in an existing storage space by cutting out a rectangle and putting a door over it.

See picture, on the right side of the companionway is what the boat came with, I would like to do the same on the left side under the drawers.

What is the best tool to make a straight cut, and how to sand it smooth. I am assuming I should be using some jig to be a guide?

Is this something I should leave to the yard to do?

[Boatguy30]

I'd use a circular saw and a jig saw or hand saw for the corners depending how thick the glass is. The door will cover the cutout anyway right?

[jbinbi]

The door will cover the hole. That doesn't mean it should look like crap. Have you done this yourself with these tools? I don't want the glass splintering or chipping along the cut. If you have done this, what type of blades have you used?

[banjoship]

Originally Posted by jbinbi

I would like to make a new cabinet in an existing storage space by cutting out a rectangle and putting a door over it.

See picture, on the right side of the companionway is what the boat came with, I would like to do the same on the left side under the drawers.

What is the best tool to make a straight cut, and how to sand it smooth. I am assuming I should be using some jig to be a guide?

Is this something I should leave to the yard to do?

On the face of it , this looks like a classic job for a multitool fitted with a high quality carbide or diamond tipped blade. Mark your cut exactly, then cut just inside your marks, say 0.5 mm. then use your multitool to sand the edges off for exact fit. With the right blades, multitools are superb at plunge cutting fibreglass.

lee

[Boatguy30]

I would use a plywood blade. You can do some practice cuts in the middle. It really seems like you're over thinking this. A multitool will work also but not as easy to get a super straight cut.

[Terra Nova]

A router, using a small diameter (3/16"-1/4") carbide bit, will produce a nice edge requiring very little sanding. Use a guide.

[P26OD228]

If possible, use a thin strip of wood as a fence. You could use gorilla tape to keep it in place. Use the blade or bit guard on a jig saw or multi tool ( rotozip ) against the fence. The fence should be offset by the distance from the guard and the blade/bit. Usually a couple of inches.
Take your time prepping it. Draw lines and look at it before the cut. The time your put into the prep will pay off in the product.

[captjcook]

A 4" diamond blade ( think "tile blade", Makita makes a good one) in an angle grinder using metal guide (double sticky tape, protect your work, lightly score full line, maybe several times depending on thickness, then plunge. Then a multi-master (FeinTool) for the corners. Sanding is minimal. Patience pays...can be done wet (mess) or a vacuum cleaner to control dust (not as effective) use a high quality particulate mask...fiberglass dust is super fine.


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[Matt Johnson]

Router, trim bit and a guide. Straight lines and just a little sanding needed. You can even do a nice round over

Matt

[sparrowhawk1]

What funjohnson said. I would go with a strait (not roundover) bit so you keep the gelcoat finish. You can make a simple jig out of plywood just make it the size of the opening minus the width of the bit plus guide on all four sides. just screw it in.

[Suijin]

Using a circular saw is a recipe for disaster. It's too big and imprecise a tool for this.

I agree that a small trim router is the way to go. OR, a rotozip or equivalent, which is probably even better as the bits are narrower so there will be less dos, less waste, and they are slightly easier to control. You'll want to fence it. What I would do is determine the size you want and then make a jig template for the outside path of the router. You'll get rounded perfect corners, if you want them, and with an outside jig there is no chance of the tool wandering outside and ruining the cut...it can only wander inside.

Benefit of router is you can make the jig the exact outline you want and use a routing bit with a bearing on it to match it exactly. Probably easier given the space constraints for a jig. Interior jig would be easy. Make it to size, out of luan ply, screw it to what will be the waste piece, and run the router around the perimeter, but be careful it does not wander.

[44'cruisingcat]

Originally Posted by banjoship

On the face of it , this looks like a classic job for a multitool fitted with a high quality carbide or diamond tipped blade. Mark your cut exactly, then cut just inside your marks, say 0.5 mm. then use your multitool to sand the edges off for exact fit. With the right blades, multitools are superb at plunge cutting fibreglass.

lee

Another vote for the multitool.

But make sure there's nothing behind that panel first!

[SV DestinyAscen]

Originally Posted by 44'cruisingcat

Another vote for the multitool.

But make sure there's nothing behind that panel first!

Another vote for dremel - might also suggest drilling boundary holes at the four corners of the cutout... for me getting that perfect cut/stop is always the hardest part.

Is your boat a lagoon 380? Somehow I remember it having cabinets on both sides?

[44'cruisingcat]

By "Multitool" I mean the oscillating saw, like the devices the medical profession used to use for cutting off plaster casts.


Very easy to cut straight lines, in fact it's difficult to cut anything other than straight lines. Drilling the corners would be very useful though.

[SV DestinyAscen]

I think we're talking about the same tool? I agree it is easy to make straight cuts, but I tend to struggle with overcut/undercut with those. Drilling corners helps make it easy to cut until there's no resistance.