Fiberglass cloth

[David Satter]

I don't do much fiberglass cloth work her in my little shop. But I have to do the bottom of a small 14 foot skiff with a new clean plywood bottom. These people are going to leave the boat in the water all summer , well probably 4 or 5 months. And it will get a small outboard. I'm thinking maybe 2 layers of 6 oz cloth. Then maybe roll on a final layer of epoxy to finish. It will get primed and bottom painted for a final coat when all's cured. Does 2 layers of cloth sound overkill? I'd like it to be pretty strong. Just though I'd get a couple of opinions. One layer of 10 oz ?

[sp_clark]

Re: Fiberglass cloth

I'd do two 6 oz. over one 10; easier to ensure the two 6's are saturated w/no bubbles.

10 oz. can present issues if you're not used to working with that weight, and it takes more epoxy to ensure proper weave-filling.

Over either I'd add maybe two fill coats, the last one gets sanded but not enough to get down to the first weave-fill coat.

Done carefully, either ought to serve for this project but again I think two light layers' outcome is more predictable than one heavy.

[Todd Bradshaw]

Re: Fiberglass cloth

I'd probably do a single layer, but it is important to note that a layer on the outside of the hull isn't going to add much usable strength to the bottom. If "strength" means prevention from damage due to hitting something like a rock or pounding under outboard power, then the outside glass will add some abrasion resistance, but not a lot else. If the goal is to prevent hitting something from breaking the bottom, then the glass should be on the inside, not the outside. Hitting a rock or pounding in waves puts outside glass in compression - something which fiberglass sheathing doesn't do all that well. The tensile strength of fiberglass is exhibited when it is put in tension, not compression. About the only thing which will put external fiberglass in tension is getting in the boat and jumping up and down. So, you can certainly glass the outside of a plywood hull, adding some abrasion resistance as well as preventing the ply from checking (which can be important on fir ply) but a serious gain in impact strength isn't likely to happen. Two layers of six ounce will wet out together in one step fine, but it is a good idea to trim off the woven selvedge edges, allowing the cloth to move and adjust better.

As far as filling the weave, trying to predetermine how many coats of resin it will take is usually a mistake with an inadequate outcome. You want to fill the weave, leaving enough additional resin on the surface that it can be sanded smooth without cutting into the glass fibers. How many coats that will take will vary with the operator's technique, the tools used, the resin brand and what state of cure the resin is in at the time. The best rule of thumb that I know of is to roll thin coats on (thin to minimize drips and runs needing to be sanded off later) rapid-fire, as soon as the layer underneath has hardened enough that you won't disturb it. You continue these coats until the weave is completely buried and then add one more coat as a sanding cushion. If that means four or five coats, so be it. The goal is to be able to sand it baby-butt-smooth without chopping up the glass cloth.

[David Satter]

Re: Fiberglass cloth

Thanks, Sounds like I'll probably do 2 layers of 6 oz. Maybe the second while the first is still green , but not wet. Not sure I'll have to see how it works out.

[shickey]

Re: Fiberglass cloth

Thanks, Sounds like I'll probably do 2 layers of 6 oz. Maybe the second while the first is still green , but not wet. Not sure I'll have to see how it works out.

Don't lay the second layer while the first is tacky (not sure if this what you mean by "green"), you will have really hard time getting the pockets of air out of it. The cloth has to be free to move while saturating the second layer. I've made this mistake, but only once.

[sp_clark]

Re: Fiberglass cloth

Don't lay the second layer while the first is tacky (not sure if this what you mean by "green"), you will have really hard time getting the pockets of air out of it. The cloth has to be free to move while saturating the second layer. I've made this mistake, but only once.

True enough though air pockets can often be 'pushed out' with gentle pressure from a brush or foamie around its edges, the inherent risk being a fold or pucker that then proves difficult to flatten out.

Depending on the brand and formula of epoxy being used, and the conditions under which it's being applied (temp + humidity) that 'green' interval between still tacky followed by 'cured enough' to allow more fiberglass cloth to be overlaid without hanging up when its saturating coat of fresh epoxy is applied and smoothed can be pretty long.

Or frustratingly short.

I've heard mention of a 'cottonball test' whereby one of those (or a Q-tip, being similar enough but more controllable) is used to gage the tackiness of the layer undergoing cure. Never done that myself, in preference for waiting long enough that I can 'ScotchBrite' the first layer of epoxy before placing the next layer of 'glass. Smoothing that is then done with either a wallpaper brush or a 'squeegee' made from the closed-cell packing-foam epoxy applicators I learned of from Todd's posts here. Done with a light touch, and free of epoxy, that makes a great tool for dealing with uncoated fabric you're trying to coax into position.

[Todd Bradshaw]

Re: Fiberglass cloth

There really is no reason to apply two separate layers of six ounce cloth. It just adds excess resin, expense and weight. Wetting out both layers at once is pretty easy and not a big deal.

[Priscilla]

Re: Fiberglass cloth

I do like what Mike is doing with the graphite/resin combo and will try it out on the Morby for abrasion resistance after the sole and garboard are glassed.
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[David Satter]

Re: Fiberglass cloth

Todd, do you mean laying both layers on top of each other dry? And wetting through both at the same time ?

[Todd Bradshaw]

Re: Fiberglass cloth

Yep, that's the way that I have always done it. For best results find someone who can mix you a continuous series of small batches of resin. Their only job is to measure them accurately and keep them coming while you apply them. It's the best way to avoid mixing errors and maximize the application time that you will have. Trim any selvedge edges off the cloth before saturation, which allows it to move and flow better into its new shape. We always put partial layers (if there are some, like cases with double-layered bottoms and single-layered sides) under the full layers, which tends to blend them in better. Resin was applied using the thin yellow foam Gougeon rollers and when squeegeed, with a slab of ethafoam packing foam about 1/2" thick by maybe 4" x 6". You want to roll or squeegee fairly slowly to avoid making tiny bubbles in the laminate and understand that it takes a little time for the glass to saturate as you are rolling.

If you encounter large bubbles, or spots where the cloth is lifting off of the wood it is because there is too much tension on it. The fix is to roll or squeegee toward the bubble, moving some excess fabric that direction to ease the tension. This happens mostly on hollow spaces, but can happen in other spots at times. It is also important to stay with the hull (or at least check frequently) until the glass has stiffened enough that it can't move. Sometimes these tension bubbles will form a while after the initial application, which looked fine at the beginning. The speed of your hardener will determine how long you need to keep an eye on things. I use fast hardener, since I learned glassing back in the days when polyester resin was all we had and a batch of it goes off in about 20 minutes, so I was used to working in that sort of time frame. You may want to use a slower hardener to increase application time. I fill with multiple thin rolled-on coats as mentioned above and I never sand resin until it has had about a week to fully cure for health safety reasons. This one used 10 oz. cloth, double-layered over the bottom inside and out.

Fur-Trade-Canoe3.jpg

[David Satter]

Re: Fiberglass cloth

I'm using Total Boat High performance epoxy. It says no amin blush, So assuming that's correct. Can I put a fill coat or second coat on to start filling the weave the next day? Or do I have to wait a couple of days for full cure and sand? I probably should have done it the same day while it was still green/tacky but didn't have time.

[Todd Bradshaw]

Re: Fiberglass cloth

Full cure doesn't actually happen in a couple of days - more like 7-10 days. Sanding before that time is a good way to get sensitized to epoxy and ensure that you can never work with it again. If you put the cloth on properly, there should be a very uniform weave texture preset with no pooling or flooded areas of excess resin on top. Any sanding before the fill coats is just going to cut into the glass fibers, which you don't want to do. Save your sanding for fully cured filler coats after a week or so. Amine blush is removed with plain old water, not sanding (it tends to quickly plug up sandpaper). With resins that claim not to blush, you should be fine adding filler coats the next day without any extra prep. If in doubt about whether any contaminants have settled on the surface, you can give it a quick water wash. It is a good idea to avoid rinsing with solvents as in many cases stray chemicals tend to add more adhesion problems than they remove.

[Autonomous]

Re: Fiberglass cloth

Green epoxy is easily cut but hazardous to sand.
I'm a big fan of using a cabinet scraper to remove runs and imperfections on green epoxy.

[David Satter]

Re: Fiberglass cloth

Sounds good Todd, That's what I'm doing and it's working out well. And I do use a cabinet scraper on runs

[David Tabor (sailordave)]

Re: Fiberglass cloth

I would suggest using Peel Ply (Release Fabric) on top of the glass for a couple of reasons.
1. it will give you a very smooth, clean finish
2. it will soak up excess epoxy
3. you won't need as many coats of epoxy to fill the weave

And here's a $$$ saving tip. Forget the expensive WEST brand Peel Ply. buy yards of dressmakers polyester dress lining material from Jo-Ann's Fabric for about 25% of the cost. It works great; I tested it meself!