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Old 22-09-2016, 10:24   #1
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Fiberglass Skin Repair-- Chamfering Difficulty

Good morning all:

I recently purchased a 1999 Leopard 38 which needs some love. One of the projects is a repair below the waterline. The pre-purchase survey indicated high moisture in a cored section below the waterline, so, last weekend, I cut into her to check out the core. The outer skin seemed relatively well-bonded (I had to pry it off with a screw driver). Also, the core felt perfectly dry. Now, unless anyone here thinks I need to take further action to determine whether the core is wet, I am ready to start the outer-skin repair.

I am having trouble figuring out how to bevel or chamfer the skin 12:1 because the skin is only about 1/4" thick. As such, the requisite bevel would be 1/12" rise over a 1" run. I am trying to create the bevel with an angle grinder loaded with a 40 grit sanding wheel. I am having great troubles figuring out much slope I am adding with the grinder and it is nearly impossible to make it uniform with this method.

Anyone out there have a good bevel/chamfer method? Thanks in advance for the help!

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Old 22-09-2016, 10:44   #2
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Re: fiberglass skin repair--chamfering difficulty

powerfile :-


Fine belt and do not rush

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Old 22-09-2016, 10:56   #3
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Re: fiberglass skin repair--chamfering difficulty

You should have the surveyor show you where it's wet, just for his learning how to use his meter if nothing else. Maybe he's willing to help fix HIS problem.
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Old 22-09-2016, 16:37   #4
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Re: fiberglass skin repair--chamfering difficulty

12:1 on 1/4" thick skin = 12 quarters = 3" inch perimeter, zero thickness at edge of 'fault' to full thickness at edge of 3" perimeter. You need to be close but you don't have to be perfect. If you're using a 7" grinder and you're new to this, better practice on a pine 2x4 or something first, you'll eat through 1/4' fiberglass and core in seconds.
If you're using a 4 or 4 1/2" grinder, it won't go quite as fast, but still pretty quickly. Use both hands with the smaller grinder too, and practice on a piece of wood probably is a good idea as well...
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Old 23-09-2016, 09:29   #5
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Re: Fiberglass Skin Repair-- Chamfering Difficulty

use a belt sander. much more controlled and you can often hook a vacuum up to suck up the dust. I prefer a 3"x18" belt compared to the link someone posted above.
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Old 23-09-2016, 11:30   #6
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Re: Fiberglass Skin Repair-- Chamfering Difficulty

Ditto on the belt sander...also, wear a full tyvek suit and facemask, preferably positive pressure. And yes, use a vacuum sander. And maybe fire your surveyor.
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Old 23-09-2016, 11:32   #7
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Re: Fiberglass Skin Repair-- Chamfering Difficulty

What a shame to have opened such a can of worms unnecessarily. I'd be real tempted to have your "surveyor" back out. I might even turn the hose on him. Hose him down real good and suggest he use his moisture meter to determine if he was wet or not. Not helpful. But satisfying.

Yes. 3 inches on a 1/4 thick inch laminate is the recommended bevel. But more won't hurt. Make sure the bevel surface is flat and not made concave by the edge of your grinder.
One mistake to avoid; Maybe you already know this, but when it comes time to start laying in your laminations, put the biggest one (and oversized) in first. Then the successively smaller and smaller diameter layers on top of it until the hole is filled. Then sand down to level and final fill and fair.

This may seem counterintuitive. But if you do it in the opposite order you will sever all the fibers of the largest cloth layer that you put on when you sand it down to level and fair. So the biggest top layer will not actually be bonded to the hull. Only to the smaller patches. I believe the Gudgeon brothers developed this technique. Lots of good info from them online.

Another technique you may want to consider is bonding the original piece that you cut out back into the hole. You didn't mention the size of the hole you cut. But this can save you considerable effort in fairing and may provide an even stiffer patch. Just cut the same 3 inch bevel on the removed piece that you cut on the hull.

You'll also want to put some thought into what resin to use. Lot's of conflicting info about resins and their relative bond strength and porosity out there, too. I'd want to know what resin was used by the builder. The last thing you want to do is create a porosity or compatibility problem where none existed before.

I would go with a 6 inch variable speed DA sander and maybe 36 grit discs. And go slowly. Any kind of high speed grinder works too fast for me to control such fine work. And the heat generated by high speed grinding can compromise the strength of the resin and fibers you are bonding to.
As far as uniformity is concerned, you don't need surgical precision or micrometer conformity here. Pretty good is good enough as long as you err on the plus or wider side. And a coarser grit will give the surface more tooth for a better bond.
And you do know to wipe down with wax and grease remover before sanding as well as after? Otherwise you grind contaminants into the surface. Lot's of people make that mistake. And grind mold release wax into the surface.
Structurally, as a bonded foam core sandwich, your hull is plenty stiff. I would be more worried about porosity than flexing. And modern foam cores and resins don't automatically delaminate the instant they get a little damp.
Personally, I think a hard plastic mallet and your ear is a better way to check a hull than a meter.
Don't obsess over it. If you screw it up, you can fix it. Over and over again as necessary!
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Old 23-09-2016, 12:57   #8
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Re: Fiberglass Skin Repair-- Chamfering Difficulty

"core felt perfectly dry"

As above.
You need a second opinion before cutting into your hull.
Radio frequency meters (non-invasive) need experience to interpret.
I always use two meters, plus corroborate with resistance pin meters
for moisture testing different materials. Need to calibrate first on known dry area.
Metal fittings behind can skew readings.
I have never had to use an IR camera, but that is an option as well to confirm.

Go to a boat yard and get a panel FG cutout and practice with that. Disk grinder is the go once you have the hang of it. Variable speed, on the thin part of the feather, next to core.
You can tape the core or put 1/6oz over just the cored area (with peelply) to give
yourself a hard datum to sand to, to avoid biting into core and creating hard spots.
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Old 23-09-2016, 15:24   #9
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Re: Fiberglass Skin Repair-- Chamfering Difficulty

Relying only on a moisture meter, is a bit risky at least, when working on cored hull. Often it is some area of not perfectly bonded core to one of the skins. I have used the following method with good success on my son's C&C sailboat: Drill 1/2 inch hole in the external upper skin section of the(aparently) delaminated or wet area, and another hole at the bottom of it. I fix a tubing into the upper hole, and start pumping with a cheap refrigeration pump. You then can see if any moisture comme out. You can warm a bit the area to vaporise any water, and check humidity level after a while. If no humidity comes out,plug another hose at the bottom hole and inject epoxy until it overflow from the succion side. plug temporary the holes and wait until the epoxy sets. It worked well for me to the big surprise of the marina 'expert'. And it is still holding well. Finish by stuffing the holes with a mixture of fiberglass soaked in epoxy.

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