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Old 01-04-2022, 08:33 PM   #1
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Fiberglass over plywood roof

Hi Everyone!

Ive just installed a new plywood roof structure to the back of my trawler. It is acting as a roof to the back deck.

I painted the roof with an oil based undercoat, and then applied a Dulux Weathershield paint to finish it off.

Whilst it looks nice, I have decided to now fibreglass the entire roof to ensure it is fully watertight and to ensure longevity.

I have never fiberglassed before, however happy to give it a try. I wanted to know - can I go straight over the existing paint? I really dont want to sand it all off if I can avoid it.

Finally - any tips on what products to use for the fibreglassing would be most appreciated, including the resin / matting to use.

Cheers!

Lyndon.
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Old 01-04-2022, 08:48 PM   #2
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Personally I would sand off the paint. It might stick to the paint but the paint wonít stick to the plywood as well as glass. More work but it will last longer. Good luck with the fiberglass work, it isnít too difficult.
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Old 01-04-2022, 10:16 PM   #3
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One word of advice.

Epoxy
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Old 01-04-2022, 10:27 PM   #4
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As said the paint needs to come off. It does not need to be perfectly clean but close to it. If not your bond will only be as good as the paint and that is not much. If tthe roof is going to see a lot of wear I would go with 12oz biax. If not 6 oz cloth will do. I prefer 2-1 epoxies like System3 but no matter what you use go with epoxy.
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Old 01-04-2022, 11:02 PM   #5
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I would use at least a couple of layers of 1708 biaxial glass. And lay it up with epoxy. Then use thickened epoxy to fill the surface. Then paint with Kiwigrip. It will cover minor defects so you wonít have to fair the deck to as fine a finish. Kiwigrip wears like iron and is very easy to apply and is a latex paint so it is easy to cleanup. Just do NOT leave the masking tape on too long or you wonít get the tape off. Pull it asap as you go around painting.
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Old 01-04-2022, 11:31 PM   #6
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Quote:
Originally Posted by Simi 60 View Post
One word of advice.

Epoxy
OK, I'll ask. Why?
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Old 01-05-2022, 01:32 AM   #7
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OK, I'll ask. Why?
Poly and vinyl esters are fine for foam and nidacore
The only real adhesive resin for timber and that includes balsa is epoxy

As for glass a 300gsm double bias is more than adequate, that's what I have on high traffic walking areas.
If it's just keeping rain out, a 180gsm plain weave is more than enough, it's what our roof has on it.
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Old 01-05-2022, 03:47 AM   #8
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Polyester resin is plastic and have very little ability to adhere. Epoxy will pull away the wood before it releases it's grip.
You have to sand. If you don't have a belt sander, go to Harbor Freight. Use very coarse belts. Buy some rollers made for rolling out fiberglass. Wear old clothes and shoes.
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Old 01-05-2022, 05:15 AM   #9
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You have to sand. If you don't have a belt sander, go to Harbor Freight. Use very coarse belts.
It's an interesting thing and I tend to agree in some instances but.....

Heavily magnified, sanding knocks over and seals the cells on timber prohibiting resin from soaking in.

A razor sharp plane leaves the cells open allowing resin in and....better adhesion.
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Old 01-05-2022, 06:08 AM   #10
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Polyester or Vinylester resins do not adhere to plywood all that well. Epoxy on the other hand, as suggested above, is an excellent adhesive. To get anything to stick the old paint will need to come off as also mentioned above.

If you don't plan to walk on it consider just using epoxy resin w no glass, a couple of coats will do, scratch it and then prime and paint. This will give you watertight and longevity, the glass is unnecessary to meet those needs. Epoxy is very flexible, a great adhesive and an excellent vapor barrier.

Glass really doesn't bring much to this type project. With Polyester/Vinylester it may even contribute to delamination from the plywood do to flexing and expansion/contraction from temp changes.

Both Polyester/Vinylester and Epoxy do not tolerate UV, and none will readily accept paint without a light sanding and a prime before paint, so scratch/prime and paint for sure.

$0.02
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Old 01-05-2022, 06:17 AM   #11
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One trick for glassing over wood I picked up from a cedar strip canoe builder but only used myself once.
I agree paint will be your enemy and needs to come off at least mostly. Also agree epoxy is preferred from my somewhat limited experience.
1st coat applied is resin only and thinned w acetone. The purpose is to get it to soak in vs cure on top of the wood. I watched and reapplied where it soaked in fastest and avoided puddles where it didn't.
2nd coat is then a conventional mix of resin or slightly "hot" with catalyst. I didn't use mat on my project but if using mat OK to include it with the 2nd coat.
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Old 01-05-2022, 07:09 AM   #12
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Polly resin is made to laminate GRP , it sticks to itself well when new..

Epoxy is a glue , first choice for most boat jobs .

To paint epoxy after it hardens , paint it while still curing and is still just slightly tacky.

IF the roof will not be walked on too often the use of quality house trailer roof paint reflects heat very well .

Any brand , just get the good , not cheap stuff.

Kool Seal Reflective Roof Coating White Elastomeric Roof Coating 1 Gallon FS
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Old 01-05-2022, 07:36 AM   #13
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Quote:
Originally Posted by Bacchus View Post
One trick for glassing over wood I picked up from a cedar strip canoe builder but only used myself once.
I agree paint will be your enemy and needs to come off at least mostly. Also agree epoxy is preferred from my somewhat limited experience.
1st coat applied is resin only and thinned w acetone. The purpose is to get it to soak in vs cure on top of the wood. I watched and reapplied where it soaked in fastest and avoided puddles where it didn't.
2nd coat is then a conventional mix of resin or slightly "hot" with catalyst. I didn't use mat on my project but if using mat OK to include it with the 2nd coat.
Iíve used this method many times and it works well. Will hold up as long as epoxy.
I learned it from LBI in Groton, Ct.
use 25% acetone and brush it on the plywood. Let it cure over night.
Then use 1 1/2 ounce fiberglass mat material and apply at least 2 coats.
Then gel coat a couple of coats.
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Old 01-05-2022, 09:32 AM   #14
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Quote:
Originally Posted by FF View Post
Polly resin is made to laminate GRP , it sticks to itself well when new..

Epoxy is a glue , first choice for most boat jobs .

To paint epoxy after it hardens , paint it while still curing and is still just slightly tacky.

IF the roof will not be walked on too often the use of quality house trailer roof paint reflects heat very well .

Any brand , just get the good , not cheap stuff.

Kool Seal Reflective Roof Coating White Elastomeric Roof Coating 1 Gallon FS
When following this advice, the OP is back exactly where he started, as his Dulux paint is that elastommeric paint that you suggest.

If this roof structure is built as strong as it needs to be, nothing more is required.
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Old 01-05-2022, 10:46 AM   #15
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The Dulux is a latex, you need to sand that off.
Use epoxy.
Thinning epoxy has been proven over and over again to be a waste of time
You do not make epoxy "hot" by adding more catalyst. The mix ratio is set.
You do need to use some sort of cloth if you don't want the plywood grain to show through. Resin alone won't prevent this.
Unless you're looking to add strength you don't need to go crazy with fabric thickness. 2 layers of around 8oz cloth is plenty
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Old 01-05-2022, 10:53 AM   #16
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[QUOTE=Bacchus;1065629
2nd coat is then a conventional mix of resin or slightly "hot" with catalyst. [/QUOTE]

You can't mix epoxy "hot", ie more hardener than called for. It's molecule to molecule chemical reaction and if there's more resin or hardener than the instructions call for, or not properly stirred, it won't harden properly. Too far off and it won't harden at all. Opposite is true for poly and vinyl resins, the more catalyst mixed in the faster the cure.
Also most epoxy suppliers don't recommend thinning the product, it degrades the properties of the resin. Many do offer a "penetrating" version for this, it's thinner right out of the bottle and can be overcoated with the standard viscosity type. Not really necessary though, standard viscosity resin will soak into plywood well enough to form a permanent bond. In this specific application, if the paint is removed with 80 or 60 grit paper, the surface will be quite rough which will help with mechanically keying the resin into the wood. If sanded glass-smooth, the penetration would not be as deep.
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Old 01-05-2022, 03:43 PM   #17
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I can’t edit my post but I’m talking polyester resin.
Not epoxy.
It can be thinned for penetration and to slow the cure.
I don’t like to add catalyst to hasten the cure because it can cause cracking if the resin is too thick.
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Old 01-05-2022, 04:55 PM   #18
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Thanks SO much to all of you for your advice!

So, just to confirm:
1. Ill sand off the paint - that seems a given!

2. The roof was built to keep the rain out, and also to jump off for my teenage boys So.... I like the idea of Kiwigrip to the top!

Therefore... the below seems the best way forward:
a) Sand
b) Clean with acetone
c) Apply epoxy resin that will soak into the timber
d) Apply matting (why not.... for extra strength) with another coat of resin
e) Do I then need to apply 1 more layer of matting over the top? Or one is enough?
f) Apply Kiwigrip over the top

Have I got this correct?

Thanks again everyone!!
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Old 01-05-2022, 05:18 PM   #19
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Don't use fiberglass mat. Fiberglass mat has a chemical binder in it to keep all the fiberglass strands together. This binder is easily dissolved by polyester resins but some mats have a binder that epoxy has a hard time dissolving, so it takes a long time to wet out. Also since mat is just loose fibers they tend to move around when the binder is removed, so you tend to get a more uneven surface then you would if using a woven fabric. Just stick to fiberglass cloth

Quote:
Originally Posted by lghawk View Post
Thanks SO much to all of you for your advice!

So, just to confirm:
1. Ill sand off the paint - that seems a given!

2. The roof was built to keep the rain out, and also to jump off for my teenage boys So.... I like the idea of Kiwigrip to the top!

Therefore... the below seems the best way forward:
a) Sand
b) Clean with acetone
c) Apply epoxy resin that will soak into the timber
d) Apply matting (why not.... for extra strength) with another coat of resin
e) Do I then need to apply 1 more layer of matting over the top? Or one is enough?
f) Apply Kiwigrip over the top

Have I got this correct?

Thanks again everyone!!
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Old 01-05-2022, 05:18 PM   #20
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Epoxy doesn’t work well with mat. Mat has a binder to keep it together that epoxy doesn’t break down well, if at all. Poly resin will break it down but epoxy not very well. You need to use cloth with epoxy. If it were me I would use 1708. It has 2 layers of cloth and one layer of mat in it. But the mat used in 1708 is sewn together not by using a chemical binder that epoxy doesn’t break down. Using 1708 will give the equivalent of 3 layers in one. But I would also use 2 layers of 1708 to make it stronger. But then I tend to overdo most things boat wise.

Edit; Sean had the same message I have just quicker typing…
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