fibreglass cloth/mat weight for repairs

[CTD]

I am planning on replacing the transom, stringers and floor on my boat. I see fiberglass comes in many different weights and was wondering what others are using. For the transom I planned on using a 1/8 inch thick mat between plywood layers to match what was there from the factory but I don't know what weight to use for installing the transom or stringers. I also noticed a big difference in prices for epoxy and was wondering what the difference is.<br />Thanks for any help<br />Dave

 

[JasonJ]

Re: fibreglass cloth/mat weight for repairs

I couldn't tell you what weight mat I used, but I know it was 1/8th inch thick. I went with multiple layers for the stringers and transom, a single layer for the floor, and used roving at the juncture points of the transom and floor, transom and sides, floor and sides. I used 30 square yards of mat and 11 gallons of Poly resin for a 16.5 foot boat. Just remember, you cannot use epoxy with mat unless it is the mat that is specific to epoxy. Regular mat is bonded together with a glue that breaks down when saturated in poly resin. You can use cloth and roving with epoxy though. Also, you can apply epoxy to poly (which is what the boat is originally mady with), but you cannot apply poly to epoxy. Just something to remember if you ever plan to try to glass something to a repaired area.

 

[BillP]

Re: fibreglass cloth/mat weight for repairs

I use:<br />Epoxy = 3oz mat and 18oz woven roven.<br />Polyester = 1.5 oz mat and 18 or 24 oz woven roven. <br /><br />Rule of thumb is one gallon of epoxy or polyester covers...<br /><br />1.5 oz mat = 32 square feet<br />3.0 oz mat = 64 " " <br />18 oz woven roven = 40 square feet<br />24 oz woven roven = 32 " "<br /><br />Poly or epoxy I generally (but not always) replace the wood same size as original and glass them in 3-4 alternating layers depending on boat. <br /><br />1. mat<br />2. woven roven<br />3. mat<br />4 woven roven<br /><br />I never put a layer of glass between the transom plys. It isn't remotely common in production boats and for the most part adds nothing that can't be done with another layer of thin wood or mechanical fasteners. Unless you have an unusual reason it is easier (and has superior clamping stength) to screw the ply sheets together for strength and do the glassing all on the inside. Don't forget the motor also clamps the parts together with massive pressure.<br /><br />Polyester resin will work fine and is easier in many ways to use but prepping requires more attention than epoxy. It is 99% impossible to sand beween the weave of 18/24oz woven roven on older boats(normally the last layer)to prep for poly. Figure this is about 30-50% of the bonding surface that will only get chemically wiped down for prep instead of "proper" sanding. Epoxy makes up for the slack here with superior bonding and higher strength. In the real world it may not make a big difference but the "math" says different. I use epoxy when dealing with a rough hull that is hard to prep. It isn't like you get a second chance if delaminations happen.<br /><br />There are a dozen ways to do this stuff and all will have the same results but with different costs and labor.

 

[CTD]

Re: fibreglass cloth/mat weight for repairs

Thanks to both of you for some really useful information. I do have a question for you jasonj, do you know why you can't apply poly over epoxy? and for you BillP would sand blasting be ok to prep prior to applying resin of any type and for what its worth my boat has 1/2 inch fibreglass transom with 1/2 inch plywood then 1/8 fibreglass mat then 1/2 inch plywood again and then another 1/8 inch of fibreglass. I don't know the difference between mat and roving but I can find out when I get ready to buy materials. Speaking of which do either of you have a favorite place for buying epoxy and polyester resins. Thanks again for the information.<br />Dave

 

[JasonJ]

Re: fibreglass cloth/mat weight for repairs

I couldn't tell you why you can't put poly over epoxy, all I know is it is one of those hard rules you have to obey. I assume its because the chemical nature of the bonding process. Epoxy by its nature provides such a superior bond anyway and can bond to a wider variety of materials, whereas poly isn't as forgiving. <br /><br />Also, one benefit of having a layer of glass between layers of wood in the transom is that it can slow water saturation and rot. Say there was a hole or gouge or whatever that you didn't know about on the transom below the waterline. It may penetrate to the first layer of ply, but it won't make it past the layer of glass to the next layers of ply. As far as adding strength, I don't know that it adds a bunch of strength, but if you run a layer of glass between layers of ply and screw the mess together with stainless screws, it will cure into a better bonded and stronger transom. I did it that way on my project and that transom will never give me problems. If you are going to do it, you may as well do it as strong as you can. Like Billp said, there are a billion ways to do it, just do it so you have a safe boat...

 

[Kenbo]

Re: fibreglass cloth/mat weight for repairs

One additional reason to alternate layers of mat and woven roving is to ensure a good bond to the existing surface. Apply the random strand mat first because of its superior adhesion characteristics then apply the woven roving to add strength. It is also important to make sure not to entrain air in the layup (Air bubbles become voids and don't have any strength). Many on this site argue that epoxy resin is better than poly. I would rather do a good job with the poly and save the extra $$$ for bait and gas. Good luck!

 

[Solittle]

Re: fibreglass cloth/mat weight for repairs

I had the transom redone by a local shop on my 23' CC a few years back as it was more than I wanted to get into. When we got to discussing the details what he was recommending he offered me an alternative. He said that he could rebuild it just the way the builder (Wellcraft) had by using sandwiched plywood OR he could use a new fiberglass plank instead of plywood of the type currently used in offshore racing boats. <br /><br />It cost a couple of hundred more but I went the fiberglass route as I never wanted to think about rot again. Unfortunately I forget what it is called. It was 1 1/2 or 2" thick, green in color and comes in 4X8" sheets. It is a composit of some sort like blown fiberglass from a chopper gun with the exception that it is not nearly as heavy as you would think. I could lift a sheet of it with no problem and I'm over 60 and not a jock.<br /><br />I hope someone jumps in with the name of the stuff for you to consider. If you find out please post it here as I would like to know.

 

[BillP]

Re: fibreglass cloth/mat weight for repairs

Sandblasting is an excellent way to prep the glass. <br /><br />According to resin companies, polyester won't bond well on epoxy. <br /><br />Instead of sandwiching glass mat between the ply sheets I use a buttery consistancy matrix of 1/2" glass fibres and resin. Trowel it on, set the wood together and screw...it squeezes out and gap fills. I do the same on the inner transom skin and screw the skin to the wood from the outside. When it kicks enough to stay put I remove the screws and seal the holes.

 

[pjc]

Re: fibreglass cloth/mat weight for repairs

for my slikker resto i'm treating all existing as well as new wood with RotDoctors CPES-clear penatrating epoxy sealer. there is an additive in cpes, an anti fungal, that kills existing dry rot fungus an protects against fungus should (when) wood is exposed to water again. (fungus--neat word--) <br />i'm using epoxy for all gluing and laming. am mail ordering from US Composits in FL. getting cloth, roving, and matting from them as well. US Comp has a sewn matting specificaly for epoxy. as some on posted, most matting is for poly use only. gotta use the sewn matt for epox.