How to cut fiberglass in a straight line

[Matt Johnson]

I typically use 3M Command strips to hold an aluminum angle as a straight edge, but for this type of cut, a simple piece of plywood cut to size and screwed in place would make a better templet.

Matt

[h2ojunke]

Some deent recommedations on here that will be you partly there.

I ran an aerospace composite shop for several years. I would recommend using a diamond grit jig saw blade for the initiall rough cut (cut within a 1/8 inch). Then use a trim router (carbide top bearing bit to ride on the fence) and fence (taped on with double sided tape) for the best results. You will have to move the fence for each cut. Be sure to leave room in the corners for a final radius (which will be the router bit O.D. If you try to jump straight to the router you will burn up your bits.

ALSO BE SURE TO CHECK BEHIND THE WALL FOR WIRING/PLUMBING/THE OUTER HULL/ FUEL TANK ETC BEFORE CUTTING ANYTHING.

[Dave22q]

Not being eager to get into the DIY game and it's just a few cuts. I would cheerfully pay a few bucks for a guy with the tools and experience to do it right. Playing with glass is a different skill set then wood working and I have no desire to invest time learning. I would rather be sailing or doing most anything else.

[topmast]

Took 3 pages before someone mentions carborundum blades in a jig saw. Would be my choice .Steady hands and don't breath the dust.

[hellosailor]

+1 on the masking tape, FRP usually has a fairly brittle surface coating (ie. gelcoat) and any tool with teeth may tend to rip off flakes. If your tool is "pushing" down against the surface that is less likely than if it is "pulling" upwards. A Japanese "push" saw made of thin metal and used for fine cabinetry cutting is good for cleaning out the corners, as is a keyhole saw with a fine toothed blade. Both inexpensive tools. You might want to pick up a scrap piece of formica or some junked boat part to make some practice cuts on, especially if you're not sure that your choice of tool will cut cleanly or quickly enough. Or, at least start with a small (i.e. 3x5) cutout, and don't make the actual full-sized cut until you are satisfied with how the tool will work for you.

[Cotemar]

My Vote would be for the Multi-Tool.

Just did a job like this last week.
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[sv.antea]

I finally found the correct answer few replies back, from the composites guy. Something I have been doing for decades. Diamond or carbide encrusted jigsaw bit (ment for cutting tile I guess) in a jigsaw. Works like a laser beam, doesn't heat up (you can run the jigsaw as fast as it will go, although slower ma give you straighter line, easy to follow straight line or go around a curve. Doesn't track off, goes exactly in the direction you push it. Last forever (almost), too. You can even buy hole saws like that. Jigsaw blades go for $6 carbide, $12 diamond.

[reed1v]

Fine woodworking table saw blades, etc. Lots of tiny closely spaced teeth. Tape where you want to cut. Go real slow. Friend uses a laser saw for just about everything now. finest cuts possible. In either case wear respirators or really well sealed eye and breathing protection. If the ultra fine glass does not give you lung cancer, the gasses will certainly rob you of your eyesight.

[otterhound]

I do this a lot and use a Fein multitool with a round high speed steel blade. It works great, is very controllable and makes very little dust. The blade is about 3" in diameter and is dark blue in color.
Rod

[SV Bacchus]

I'm with the 2 or 3 others that are asking the basic "What is behind there now?" They didn't use those words but it is what they were asking.

I would think if it was usable space the boat builder would have utilized it. Not likely they would pass on usable space just to save money on not installing a door. Lots of folks look for mucho storage when shopping so it is in their interest to use all space available.

Check closely for hoses and wiring!

[captmikem]

no matter what tool you use, a fence or guide will do you right. Easiest is to attach it to the piece you are going to throw away, the cutout. Simply drill and screw or bolt your guide or fence to it.

M

[ramblinrod]

Originally Posted by ramblinrod

If your boat is of high value, you are not practiced in this type of work, and you lack confidence in your ability, you should first practice on a piece of scrap until confident you will not wreck your boat, or hire it out to a professional.

This is how I would do it.

1. Place masking tape over the area to be cut to reduce surface splinters, and tool marks.
2. Draw lines on the tape with black magic marker.
3. Cut to the inside of the marker line with whatever tool(s) you have that will do the job (circular, jig, reciprocating, oscillating saw, dremel, or roto zip).
4. With a random orbital, sand the edge rounded, just to the other side of the marker line.
5. Pull the tape off on an angle toward the cut edge.
6. Dress the edge with gelcoat or paint to match the other side.

Note that FRP is tuff $#!+ and will dull regular carbon steel tools quickly. Carbide tipped tools will last much longer.

Whoops! One more tip. Leave a radius in every corner equal to the substrate thickness. (FRP does not like sharp corners and cracks will tend to radiate from them if any stress is present.) Also the reason why all drilled holes should be chamfered, especially on the gelcoat side.

[capt. enki]

I have used a 4" plunge circular saw with a laser guide with good results to cut the longer straight cuts and either a rotozip or multitool to finish the cuts. Drilling the corners eases the end cuts