Laying Fiberglass Dry

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Sep 9, 2018
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Asheville, NC
#1
So, I’m finishing up my tray and I was having trouble getting my 1708 to wrap around a 90 degree corner. I called up US Composites expecting to buy some thinner mat to make the corner but the guy I spoke with asked me my procedure for laying the glass. I explained to him in short, that I wet the surface the glass will be applied to, then I have a large pan that I pour some resin onto, then lay the glass onto that pan and wet it with resin before installing to the hull. This is what almost everyone has told me and just about everything I can find online explains a similar procedure. The gentleman at US Composites said that wetting the mat before installation was a mistake. He said that it is wasteful and lots of people do it that way, but it’s wrong. I think he thought I was trying to argue to him, but I was just saying I had never heard of that. He said that in order to get the mat laid down properly, I needed to apply a medium to heavy coat of resin on the surface that I am applying the glass, wait 30-40 minutes until the surface is tacky THEN lay the DRY glass onto the tacky surface and push it into place. Wait another 20-30 minutes and mix a new batch of resin and wet the rest of the mat. I’m not doubting anyone’s technique but does anybody else lay glass down dry and then wet it? I’m still new to this and finishing up my tray. What is your experience with getting fiberglass to wrap around a 90 degree corner?
 
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Joined
Feb 27, 2012
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LOTO
#3
A year ago I was in your exact position, they (probably the same guy) told me the same thing. I did it that way and it turned out great. The hard part for me was having the patience to wait and press the mat into place. If it's not tacky, the mat will move/slide or pull up around a 90 degree bend.
 
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#4
He even want as far to say that the mat WITH backer would stick even better because of the chopped strand side. I used mat with no backer for this section because I thought it would conform easier! No problem though, I’ll get it right I just wanted to hear some opinions
 

SXIPro

JM781 Big Bore
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#5
I was watching a boat building video the other day in preparation for me putting my SN back together and they did just that...wet the boat with resin, let it tack up a bit, lay the cloth how you like it, and then slap the resin on the cloth.
 
Joined
Sep 9, 2018
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#6
He did mention that wetting the mat obviously lets it slide around and sometimes makes the corners lift. Luckily everything I’ve done so far has been on pretty flat areas so I didn’t have any trouble with the mat lifting. I’m sure I’ve used way too much resin on my build so far but... too late! Now I know :cool:
 
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#7
The thing you really have to be careful of is using too much resin. The use of too much resin actually makes it weaker not stronger. You only want enough resin to wet on the the glass, nothing more. If fact the correct amount of resin is actually way less than most people think it should be.

I find what works best for me is to have a foldable table in the garage covered in plastic. I then layout a sheet of glass and pour on resin then spread the resin and force it into the glass with a plastic Bondo spreader. I then use the Bondo spreader like a squeegee and get as much resin out of the glass as I can. I then lift the piece of glass off the table and place it onto the ski. Then repeat with the next sheet of glass. The resin remaining on the table holds the next piece of glass in place when you start squeegeeing again.
 
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Sep 9, 2018
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#8
The thing you really have to be careful of is using too much resin. The use of too much resin actually makes it weaker not stronger. You only want enough resin to wet on the the glass, nothing more. If fact the correct amount of resin is actually way less than most people think it should be.

I find what works best for me is to have a foldable table in the garage covered in plastic. I then layout a sheet of glass and pour on resin then spread the resin and force it into the glass with a plastic Bondo spreader. I then use the Bondo spreader like a squeegee and get as much resin out of the glass as I can. I then lift the piece of glass off the table and place it onto the ski. Then repeat with the next sheet of glass. The resin remaining on the table holds the next piece of glass in place when you start squeegeeing again.
Should I be concerned that I used way too much resin for reinforcing the hull?
 
Joined
May 12, 2010
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Charlotte, NC
#11
the purpose of letting the resin "gel" is to allow any air bubbles to release. if you lay a new piece of cloth over the resin before it gels, you can trap air bubbles inside the part your making and making it weaker. laying a solid base coat of resin will wet out most light weight cloth. i try not to use really thick cloth or biaxial with the mat because it takes soo much resin to wet out and the mat is harder to work with. i also prefer the faster drying resin because of how long it takes to gel.
 

smoofers

Rockin' the SQUARE!!!!
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#14
The only time I pre-wet my cloth is if it's something super thick like 1708 with CSM and I'm laying epoxy. In those instances, pre-wetting the glass makes me spend way less time trying to get the cloth properly wetted out. Most thin cloths like 8oz twill are so delicate that if you pre-wet them you'll destroy your piece just trying to pick it up wet.

Also, tacky resin holds glass in place much better than fresh. The guy at US Composites is spot on.
 
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#15
The only time I pre-wet my cloth is if it's something super thick like 1708 with CSM and I'm laying epoxy. In those instances, pre-wetting the glass makes me spend way less time trying to get the cloth properly wetted out. Most thin cloths like 8oz twill are so delicate that if you pre-wet them you'll destroy your piece just trying to pick it up wet.

Also, tacky resin holds glass in place much better than fresh. The guy at US Composites is spot on.
This has been an awesome learning experience! I’m getting better at it everyday. Wet mat will fall apart and stretch very easily, I found that out the hard way haha.
 

smoofers

Rockin' the SQUARE!!!!
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#16
To add to my earlier post, I don't use the tacky resin trick that often. Most times I'll just wet the entire surface I'm glassing with resin and immediately lay down the first layer of cloth. Even freshly mixed resin does a pretty good job of holding glass in place to finish wetting it out. On thick layups like 1708, a fin roller is a great tool to help wet out the glass without overloading it on resin. The trick with a fin roller is to slowly roll it across the glass - you'll see the wave of resin or air as the roller pushes it. Speed wise, it should take you about 3-4 seconds to travel 12" with the roller. Don't use it like you would a paint roller and quickly roll back and forth. When used properly, you'll be amazed how little resin you can get by with to properly wet out your glass.
 
Joined
Sep 6, 2016
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White lake, Mi
#17
So the way we do it at my Formula SAE shop "best way" is to pre wet the cloth and then lay it down. However, as it was previously stated to much resin will make your lay up weaker. the strength actually come from the fiber tows and orientation, not the resin. Once your laminates are down you then utilize the proper consumables and a vacuum bag/vacuum to pull all the access resin out of your lay up.

However, that method gets pretty costly using consumables, vacuum, etc. When I redid my tray a couple years ago (before I had any composites knowledge lol). I did the other method of post saturating the cloth. That worked well and has held up to a lot of tough riding. Just make sure that the cloth is completely down and there is not any air bubbles or voids between the two surfaces.

Hopefully this helps clear up the confusion between the two methods you saw. The other method would not be worth spending the money on all the other items unless you plan on doing composite frequently.
 
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