Fiberglass over plywood roof

[Comodave]

One of the nice things about Kiwigrip is that it will hide many minor defects in the finish before you paint. So you don’t have to put in lots of time getting the deck completely fair, just close. It will hide a multitude of sins…

 

[jgwinks]

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.

That's important for critical bonding jobs like scarf joints, a cut or planed surface bonds better than a sanded one. But planing down a large area like this would be a horrible job, and all you're trying to do is stick the glass to the surface. There's virtually no load trying to peel it apart. Sanding is adequate here.

 

[lghawk]

Hey Simi!

Mate re the Emerclad - how do you apply this? With a special roller, to get the texture and to create the non-slip?

Cheers!

L

 

[Simi 60]

Hey Simi!

Mate re the Emerclad - how do you apply this? With a special roller, to get the texture and to create the non-slip?

Cheers!

L

Textured foam roller

 

[lghawk]

Thanks mate!

Ive said this on here before but Kiwigrip seems nothing more than an acrylic waterproof membrane as used in the building game.

Same same but different variants here, no pictures of boats on cans

https://www.google.com/search?q=acr...CHBF_en-GBAU945AU945&sourceid=chrome&ie=UTF-8

The one I and others in the boatbuilding game have used here emerclad
I have been using this on and off for over 30 years
https://www.emer.com.au/products?f[0]=solutions:918

Textured foam roller

 

[Painless]

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.

Go to YouTube and watch some of the early videos of "Boatworks Today" Andy has a video on nearly every question you could ask.

 

[FF]

"Sanding is adequate here."

You bet , since there will be some material like CSM or light glass over the surface a really rough surface will have great sticking ability , and still look fine on top.

I prefer #16 or #24 floor sanding paper , its silicone carbide so you will only need one piece , and its FAST!

 

[Circumnavigator]

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

Agree completely. West System Epoxy is made to seal the wood -- in modern wood boat construction it is used without glass. Glass adds nothing to the deck -- it is much stiffer than the plywood -- it's like trying to use a steel cable and a nylon rope to hold up a load and expecting to get the strength of the two added together. Scratch the paint hard with 36 grit sandpaper and apply two coats of West System. Then paint it -- as noted above, the epoxy does not like UV.


Jim

 

[lghawk]

Hey Everyone,

Thankyou all for the advice.

I have decided to go with the Double Bias matting and have decided to "experiment" first on my sunlounge at the front of the trawler. It is 2400mm x 1800mm. I am going to also do the sides of it, however will tackle the top first.

My question is - how do I calculate how much epoxy I require? I will be doing 1 x sealer cost (after I sand off the paint) and then 1 coat (at least) when I lay down the matting.

Can anyone help with this?

Thanks guys!

 

[DNT99611]

A really good hand layup will have the same weight of resin as the glass. Most will not do that well so I would weight the glass and have double that weight in epoxy for the job. You will see a lot of folks saying use 1708 but for me, with epoxy, I would use 1200 or 1700 biax. Much easier to get a clean job with no air in the laminate.

 

[lghawk]

Thanks so much!

So, using a metric system, am I correct in saying:

2.4m x 1.8m = 4.2sqm.
The matt I am looking at is 400grams/sqm.
If I double that I will need 800ml of epoxy psm.

Does this sound correct?

 

[DNT99611]

Sounds right to me. And I would not worry about ending up with extra epoxy. Unlike poly the shelf life is really long. I had some System Three resin that was 25 years old that worked like a champ. But it was kept at a steady 60f the whole time.

 

[syjos]

Two reason why I apply 10 oz glass fabric over plywood decks and roofs with epoxy.

Puncture resistance. Epoxy and wood alone will not prevent a dropped screwdriver, winch handle etc from penetrating the thin epoxy skin and creating a path for water.

Consistent epoxy thickness. It easy to sand off too much epoxy from the surface so I never know how thick the epoxy ends up. I use the FG cloth as a thickness gauge with benefits. The epoxy is applied to the FG cloth with a squeegee and the cloth pressed tight to the plywood with a serrated FG bubble roller. While sanding, I get a visually warning when the sander reaches the surface of the FG. The epoxy film thickness will never be less than cloth thickness.

Prevent print through. Different materials like plywood, wood plugs and epoxy based filler all expand and contract at a different rate causing unevenness of the surface which will print through the epoxy coating over time. FG cloth being less flexible and thicker will reduce print through. Avoiding dark colored paint on roofs and decks will also reduce print through from reduced surface temperature.

 

[syjos]

To insure adequate penetration of the plywood surface with standard epoxy, I heat the surface with a heater or two tented with a tarp. When the surface is 75 - 90 degrees, mix the epoxy and start rolling onto the warm surface. Turn off the heater. As the plywood cools the cells within the plywood contract and will draw the epoxy into the wood.

When epoxy is applied to a cool plywood surface, the exothermic heat from the curing epoxy warms the wood which will cause it to expand and release trapped air from microscopic wood cells. Instead of drawing epoxy in as in the previous method, the air released from the warming wood pushes epoxy out creating bubbles.

On new wood or wood in good condition, standard epoxy is used for the first coat. On badly rotted wood, I use Smiths penetrating epoxy for the first couple coats and follow up with standard epoxy.

 

[Fish53]

As others suggest use epoxy, I recently did the deck on my boat using one layer of biaxial cloth and it came out well. One trick a friend who does this for a living told me was to use those cheap yellow squeegees to spread the epoxy, forces the epoxy into the cloth and provides a smooth even application.

 

[rslifkin]

As others suggest use epoxy, I recently did the deck on my boat using one layer of biaxial cloth and it came out well. One trick a friend who does this for a living told me was to use those cheap yellow squeegees to spread the epoxy, forces the epoxy into the cloth and provides a smooth even application.

I'll second the yellow squeegees. Those things have been great while laminating fiberglass and epoxy over foam core for end plates and baffles in my new water tank.

 

[garbler]

Seems odd that so many experts have failed to address a few very important factors in a plywood hardtop. If your intent on overlaying a plywood panel with GRP your primary consideration should be the quality of your ply. Any secondary bond on plywood is only as good as the top laminar or veneer. You can lay down all the resin systems and fabric you want but if the surface veneer let’s go what do you have, a sheet of GRP with lots of wood stuck to the under side. A mess really. You need to use top quality ply with solid layups like a BS 1088 class. When good builders tab a plywood bulkhead they grind back the bonding edges the full width, or more, for the fabric tapes. Anything less results in loose bulkheads with water migration and rot.

As previously mentioned medium to thicker epoxy resins are hard to wet out. Most fabrics float in epoxy which is the reason that Dynel
( polyproplene) epoxy decks require special techniques. Secondly on a big flat horizontal surface where sunlight and UV exposure is the maximum you can count on UV degradation and epoxy breakdown unless you factor that in with top coatings. Vinyl ester resins will not buy you much with this type of job so polyester resin may be your best option. Either way a good UV protecting paint is your best friend but just any marine paint by itself will not prevent UV damage for any long term solution depending on your location and sun exposure. An easy answer that I’ve used many times when I was in the business is to prime coat with a good quality silver paint. The silver is reflective hence much cooler and a proven method. Your biggest concern with a big hardtop like you want is to, (1) sufficient camber for drainage and (2) spend the time and expense in protecting the end grain or edges as failure and moisture migration starts there.

Good luck
Rick

 

[Simi 60]

Seems odd that so many experts have failed to address a few very important factors in a plywood hardtop. If your intent on overlaying a plywood panel with GRP your primary consideration should be the quality of your ply. Any secondary bond on plywood is only as good as the top laminar or veneer. You can lay down all the resin systems and fabric you want but if the surface veneer let’s go what do you have, a sheet of GRP with lots of wood stuck to the under side. A mess really. You need to use top quality ply with solid layups like a BS 1088 class.

BS1008 is not what it used to be.
I have used gaboon variants before, hand selected.
When selecting, 2 in 3 were rejected.

Recently I bought a dozen sheets of what appeared to be great quality 5/8th BS1088 hardwood ply, had it stored onboard for ongoing repairs.

I cut a piece off, left it exposed on the duckboard for a week or two getting regular sun and salt water and it fell apart in sheets.

Sold it off cheap as workbench/bookcase ply and replaced it with AS2271 for our latest project.

 

[koliver]

Sounds right to me. And I would not worry about ending up with extra epoxy. Unlike poly the shelf life is really long. I had some System Three resin that was 25 years old that worked like a champ. But it was kept at a steady 60f the whole time.

In 1992 (+- 1), I hit a rock with the front bottom corner of my sailboat keel. For the repair I bought a pack of cold cure epoxy. I have it still, 30 years on, and I have used it this past year with great success. It has been kept in my garage the last 10 yrs or so, with temp fluctuations consistent with outdoor highs and lows in SW BC.
I see no limit to its shelf life.

 

[syjos]

Seems odd that so many experts have failed to address a few very important factors in a plywood hardtop. If your intent on overlaying a plywood panel with GRP your primary consideration should be the quality of your ply. Any secondary bond on plywood is only as good as the top laminar or veneer. You can lay down all the resin systems and fabric you want but if the surface veneer let’s go what do you have, a sheet of GRP with lots of wood stuck to the under side. A mess really. You need to use top quality ply with solid layups like a BS 1088 class. When good builders tab a plywood bulkhead they grind back the bonding edges the full width, or more, for the fabric tapes. Anything less results in loose bulkheads with water migration and rot.

I would definitely use 1088 for building hulls and other more critical use but for cabin sides, roofs, bulkheads, I use less expensive Sande/Verola marine plywood.

Prior to using any previously not used plywood on a boat, I perform the Gougeon Brothers boil and bake test on a scrap piece. On new plywood, I laminate a strip of scrap 1708 FG with epoxy to a piece of plywood with a tab sticking out and let it cure for a week, then try to pull it off with vice grips to see where the failure occurs. It surprised me when one sample of a Lloyd's approved 1088 plywood failed the Gougeon test around 30 years ago. I'm sure the consistency of marine plywood quality is much improved now.

Good builders also use foam or some other material to widen the contact area between hull and bulkheads to spread the load and eliminate hard spots. Sort of a wide fillet that is tabbed over.

 

[DNT99611]

Good builders also use foam or some other material to widen the contact area between hull and bulkheads to spread the load and eliminate hard spots. Sort of a wide fillet that is tabbed over.

It takes a little more time to do it that way but that is offset some by not having to built large fillets. For sure a good way to do it.
I like working with foams like divinacell and lately have gone to using honeycomb in place of plywood in a lot of places. Good plywood has become hard to get and you can never be sure if it is what you think it is.

 

[syjos]

It takes a little more time to do it that way but that is offset some by not having to built large fillets. For sure a good way to do it.

Most boatbuilder don't seem to make fillets or use foam blocking at the hull/bulkhead joint. They use tabbing only.

As I said earlier, only the top boat builders pay attention to details like that and most recreational boats don't require it for their usage.

I'm sure Nordhaven, Krogen, DeFever and other top boatbuilders would painstakingly apply those details in an ocean going boat.

 

[garbler]

Interesting comments but on the whole you’re missed my points. First BS 1088 is not a brand of plywood but a manufacturing spec so there are literally dozens or hundreds of mills laminating to these specs. Like anything else there are some companies that do it better than others. The best I ever used was from two mills in Israel so you just can’t make a statement that 1088 is no good.

My point with bulkhead tabbing had nothing to do with hard or soft fits, solid lam or cored hulls and cushion tabs. It was all about the need to strip and prep the ply for reliable secondary bonds. Some of you guys are like Kudzu you just take off in every direction and eventually smother the message

Regards

 

[Lou_tribal]

First thing I would suggest is to know what you want to achieve.
Do you want to add structural strength or only waterproofing?
If just water proofing there is no need to waste your money on expensive fb cloth.
If just waterproofing it can be as simple as applying the proper primer and paint without the hassle of going with epoxy as it is a roof and not underwater and wood can be directly paint if done properly. More coats of primer and paint properly done will give better result than epoxy and less coats if not done properly, not even talking about cost.
The Ferrari would be epoxy, many alternate layers of mat and cloth, primer, sand, primer, sand etc until perfect flat reached than 2 part epoxy paint, but question is do you need to pay for a Ferrari to run at 20 miles an hour?

L

 

[syjos]

Some of you guys are like Kudzu you just take off in every direction and eventually smother the message

Regards

Hmmm.... I've learned a lot on this forum over the years, a lot from comments and discussion that wandered off the original topic. That's the way it seems to go on most forums.

 

[garbler]

The previous poster ( Lou Tribal ) has cut to the chase telling us that all you really need is a reliable water proof membrane. If you build this top so it’s removable for maintenance or framed and skinned enough to support your weight then you can recoat as necessary. Remember you need camber for drainage and strength.

When I was in the business back in the 70’s my shop used a product called
‘ Arabol ‘ not only on coach roofs but upper deck surfaces where heavy foot traffic was not a factor. We must have done five or six charter/ half day boats a year for three years. Even after ten years of So Cal and Mexican waters the Arabol held up. Our procedure was to staple down fiberglass screen first then pour out trowel then brush a heavy coat to fill and bury the screen. It’s self leveling so a smooth surface is easy. The product was invented by Borden’s by chemist Henry Arabol. It’s a dairy product so no volatiles or smell it’s completely harmless. It has a Mil-spec # and was used by the USN years ago. Fire proof so it’s NFPA compliant and will not mold. It’s a high gloss white but paintable with long oil alkyd enamels. Given it’s history it appears to be more or less UV resistant. Cheap, easy to apply and clean up and water proof.

Thorpe used to sell the stuff as pipe and exhaust lagging adhesive then found the marine market and labeled it ‘ Easy Deck ‘. I think they went out of business but my old contacts say Childers makes a product called ‘ Chill-Seal ‘ which is the same thing. Product number is CP50A HV2. This is an easy answer to your job

Good luck
Regards Rick

 

[syjos]

First thing I would suggest is to know what you want to achieve.
Do you want to add structural strength or only waterproofing?
If just water proofing there is no need to waste your money on expensive fb cloth.
If just waterproofing it can be as simple as applying the proper primer and paint without the hassle of going with epoxy as it is a roof and not underwater and wood can be directly paint if done properly. More coats of primer and paint properly done will give better result than epoxy and less coats if not done properly, not even talking about cost.
The Ferrari would be epoxy, many alternate layers of mat and cloth, primer, sand, primer, sand etc until perfect flat reached than 2 part epoxy paint, but question is do you need to pay for a Ferrari to run at 20 miles an hour?

L

As I described in post 43, the purpose of epoxy and FG is not for strength. The FG/epoxy sheathing is for puncture resistance from dropped objects, to help achieve an even epoxy thickness and mitigating print through. And it does make the roof truly waterproof.

Why would primer and paint alone achieve better results than epoxy and fabric?

And if more coats of primer and paint is adequate for roofs and decks, why for centuries was canvas applied to boat decks and roofs and painted prior to the proliferation of glass fabrics and resins?

 

[garbler]

Epoxy certainly excels in more jobs than not but there are some drawbacks such as price, ease of handling and it’s worse failing especially in a horizontal sun catching roof top and that is it’s propensity to break down from UV rays. Therefore a good UV protectorant silver paint and a top coat is essential. I’ve laid lots of canvas decks years ago but back in the 60’ and 70’s we dropped this procedure because good quality long strand Egyptian or equivalent bolts of cotton duck were no longer available. The good canvas decks were laid in white lead ( try to find it ) dampened then clamped and stretched tight. As the the cotton shrunk it pulled tight and gave you a great flexible surface to paint — no undercoat ! The fabric has to be carefully laid out and cut so the warp runs the long way as shrinkage is more across the width. Our canvas clamps had duck bill jaws with tensioning screws to pull the job smooth.

The reason canvas laid decks were so common years was they were laid on planked decks and coach roof where the planks swelled and shrunk up to 8-14% across the grain. These old decks were commonly clear white pine or white cedar/juniper. They were tight fitted planks with no caulking seam. Canvas really won’t work with oily hot teak decks in the tropics and caulked seems as fastener bungs will always print through. The canvas decks would stretch and stay down. They were then painted and recoated as necessary. The smart move was to thin down the paint 20% or more to avoid heavy paint sick decks. When you had too much old paint they became paint sick and cracked and a new deck was necessary. When plywood decks showed up canvas went away as fiberglass resin and epoxy were the smart choice. These days a good epoxy and Dynel deck is the way to go as the previous poster noted is can withstand impact punishment and foot traffic much better. I really don’t see dropping tools or puncturing a hardtop as much of a risk but who knows. Arabol on the other hand is even more flexible, less likely to puncture except sharp or pointed tools and can be easily patched.

 

[syjos]

Epoxy certainly excels in more jobs than not but there are some drawbacks such as price, ease of handling and it’s worse failing especially in a horizontal sun catching roof top and that is it’s propensity to break down from UV rays. Therefore a good UV protectorant silver paint and a top coat is essential.

I paint epoxy coated exterior surfaces with white LPU paint. The color white reflects the most sunlight and has the coolest surface temperature. Surface temperature of silver paint, aluminum coatings and other "reflective coatings" are 20 plus degrees higher than white.

I would guess that silver undercoat with a white topcoat on a roof would yield the coolest interior temperature but just white paint works great at reflecting sunlight.

 

[Mako]

Quote: Originally Posted by garbler View Post
Some of you guys are like Kudzu you just take off in every direction and eventually smother the message

Hmmm.... I've learned a lot on this forum over the years, a lot from comments and discussion that wandered off the original topic. That's the way it seems to go on most forums.

@Garbler, I appreciate your technical comments, but the TF isn't a white paper. It's not your masters thesis. It's a place for people to chat and BS and tell fish tales. And as @Syjos stated, besides being entertained you can learn a lot from cross-conversations.

... and speaking of waterproofing a roof... yesterday in the thick grey mud outside Canaveral Locks, my 5lb Danforth way, way outperformed my 22lb CQR which had a hard time penetrating