Cracks In Sheathed Plywood Decks

[starcruiser]

Plywood cruiser with fibreglass sheathing.

Two years ago I repainted the decks, first sanded them and then applied two-pack epoxy glue to numerous hairline cracks in the sheathing.

Then finishing with anti-skid paint.

It all looked good, but now the hairline cracks have reappeared.

I guess the best way to tackle them would be to sand everything back to bare wood, apply new sheathing etc. Too much work and I haven't the time.

Can I just sand back the cracks, tape over them individually with strips of fibreglass cloth, and epoxy?

[Don C L]

Some photos would be helpful. How many are there? Where are they? Are they causing any leaks you can see? Any evidence of water intrusion? You can glass over cracks but I am curious what is causing them. You could sand down to expose the glass layer and apply a new layer or two as long as the wood has not had water intrusion, I'd say.

[Snowpetrel]

Yeah, don has it right.

Ideally you redo the whole lot, but you might get away with patching areas. You really need to find the cause of the cracks, is it flex, water intrusion, or just old age. Your previous fix didn't add any real strength back, so was bound to fail at some point.

If you can lift the edges of the existing class near the crack see if it can be peeled back easily.

If so you really need to get back to well adhered glass before repairing and check the condition of the wood underneith. I am just going through all this with a 23 foot plywood trailer sailer...

[South Pacific]

If you just re-glass where the cracks are they will open up again. Sand back a couple of inches each side of the crack, cut a groove about 1/2 the depth of the ply along the join (a 1/2” router bit would be ideal), glue a wood fillet in the groove, fair and re-glass. If using a router, use an old bit as you will probably hit a nail or 2 along the way.

[UNCIVILIZED]

Without opening them up a bit to find out what's happening, & why, you'll never know for sure, but... Odds are, the decks aren't stiff enough. So that given time, & flexing, the cracks are reappearing. Though, regardless, you need to check for water intrusion (& rot), as mentioned.

You might be able to sneak by with taping the cracks with carbon fiber tape (as it's stiffer), & sealing the tape, well. But it's risky, & you could be faced with this issue again in a couple of years (or less).
BTW, if you go this route, seriously consider doing so in conjunction with added deck stiffeners on it's underside, denoted below.


Aside from showing us some pics, it would help a lot if you told us:
- How thick the current plywood, & also the current glass is.
- How much, measured, camber there is to the decks. As in inches (or fractions there of) per foot of width/beam).
- The spacing underneath of the deck, in between the; beams, bulkheads, & stringers. AKA the panel sizes.
- Whether or not there's glass on their undersides. As with thin decks, this can make a surprising difference.


And the ugly fact may be, that you'll need to strip the current glass, via a; heat gun, paint scraper, pliers, & a grinder.
Then stiffen them up a bit, say with another layer of plywood, glued atop them. Or via adding a core on top of the current ply (after checking for moisture & rot), & then a layer of ply or glass atop of the core.
---> The new ply layer being one where all of the seams/joints are properly scarfed. <---

Also, a few other options to add to their stiffness, are;
- Adding a structural ceiling belowdecks.
- Adding more framing underneath, to support them.
- Adding a laminate bulking layer, prior to adding a new, thicker, layer of glass on top.

Some guys will glue in tongue & groove ceilings, or tongue & groove ply (that's made with waterproof glues). Or some pretty, strip planked ceilings.
While others will add more ribs; followed by ceilings of various types overtop of the new ribs (framing).

And if you decide to add an internal ceiling or ribs, it's advisable to add them first. As odds are you'll be using screws to hold them in place while the glue cures. Thus adding a few new holes to your deck. Which you'll fix, when you redo the deck's covering.


Speaking of which, it's probably wise to add a more substantial layer of glass than is currently in place. Say 1/8" (3mm) thick, minimum. And 5mm+ would be better. If the current ply's thin/the decks flex a lot.

Stiffness (essentially) varies with the cube of the thickness. So that a deck which is twice as thick, is 8x stiffer. Thus, a small addition to such, goes a long way. Even just going from 1 layer of 8oz cloth, to 1.5mm-2mm of glass, can help a lot.
Especially if flexing's a problem. Which it sound like it is.

[South Pacific]

Originally Posted by UNCIVILIZED

Without opening them up a bit to find out what's happening, & why, you'll never know for sure, but... Odds are, the decks aren't stiff enough. So that given time, & flexing, the cracks are reappearing. Though, regardless, you need to check for water intrusion (& rot), as mentioned.

You might be able to sneak by with taping the cracks with carbon fiber tape (as it's stiffer), & sealing the tape, well. But it's risky, & you could be faced with this issue again in a couple of years (or less).
BTW, if you go this route, seriously consider doing so in conjunction with added deck stiffeners on it's underside, denoted below.


Aside from showing us some pics, it would help a lot if you told us:
- How thick the current plywood, & also the current glass is.
- How much, measured, camber there is to the decks. As in inches (or fractions there of) per foot of width/beam).
- The spacing underneath of the deck, in between the; beams, bulkheads, & stringers. AKA the panel sizes.
- Whether or not there's glass on their undersides. As with thin decks, this can make a surprising difference.


And the ugly fact may be, that you'll need to strip the current glass, via a; heat gun, paint scraper, pliers, & a grinder.
Then stiffen them up a bit, say with another layer of plywood, glued atop them. Or via adding a core on top of the current ply (after checking for moisture & rot), & then a layer of ply or glass atop of the core.
---> The new ply layer being one where all of the seams/joints are properly scarfed. <---

Also, a few other options to add to their stiffness, are;
- Adding a structural ceiling belowdecks.
- Adding more framing underneath, to support them.
- Adding a laminate bulking layer, prior to adding a new, thicker, layer of glass on top.

Some guys will glue in tongue & groove ceilings, or tongue & groove ply (that's made with waterproof glues). Or some pretty, strip planked ceilings.
While others will add more ribs; followed by ceilings of various types overtop of the new ribs (framing).

And if you decide to add an internal ceiling or ribs, it's advisable to add them first. As odds are you'll be using screws to hold them in place while the glue cures. Thus adding a few new holes to your deck. Which you'll fix, when you redo the deck's covering.


Speaking of which, it's probably wise to add a more substantial layer of glass than is currently in place. Say 1/8" (3mm) thick, minimum. And 5mm+ would be better. If the current ply's thin/the decks flex a lot.

Stiffness (essentially) varies with the cube of the thickness. So that a deck which is twice as thick, is 8x stiffer. Thus, a small addition to such, goes a long way. Even just going from 1 layer of 8oz cloth, to 1.5mm-2mm of glass, can help a lot.
Especially if flexing's a problem. Which it sound like it is.

It's more likely that the cracks show because the ply was butt jointed at the beams instead of being scarfed, hence a simply fix as I mentioned above is all you need without taking weeks and adding a lot of weight rebuilding the deck.
Check the condition of the ply as there may have been some water ingress.

[UNCIVILIZED]

Originally Posted by South Pacific

If you just re-glass where the cracks are they will open up again. Sand back a couple of inches each side of the crack, cut a groove about 1/2 the depth of the ply along the join (a 1/2” router bit would be ideal), glue a wood fillet in the groove, fair and re-glass. If using a router, use an old bit as you will probably hit a nail or 2 along the way.

So then, "spline" them from on top? Followed by new taping.
It might well work. Have you tried it, & if so, what were the specifics of the deck's construction, & it's supporting structure underneath?

And a variation on it, could be to route a channel into the decks, along each crack. Deep enough to hold several layers of tape, so that it doesn't stand proud when you're done.
Even to possibly include, making the channel a stepped or beveled one, so as to accomodate each, successively wider layer of reinforcing tape.
Again, allowing you to add several layers of tape, without adding thickness to the deck.

Albeit, even with these options, I'd also prefer to add more stiffness to the deck via it's underside. But they're definitely viable ideas.

To the OP. Is it safe to assume that most of the cracking is in areas where there's non-skid in the deck's finish? So that after re-fixing the cracks, you don't have to worry much about making things mirror smooth, for re-painting?

[starcruiser]

Thanks everyone for the helpful advice.
Unfortunately I am quite a long way from the boat but will take and post pictures when I next go there.

The plywood deck thickness I estimate at 1/2 inch maybe a little more. Beams under the largest area, the foredeck are at 1 ft centres and side decks are very narrow, maybe 8 inches but I didn't know what's underneath that area supporting nor whether the joints are scarfed --- but good concerns raised and a close inspection is warranted when I go up to see.

The cracks are not that many, and in random places on the foredeck and side decks.

Yes, on the anti-skid paint.

The cracks are very minor indeed...hairline, not necessarily straight, spidery. The sort of ones so narrow that sometimes we are tempted to paint over them hoping the paint will fill them.

Luckily there is no water inundation.

I suspect the cracks are the product of age more than anything else.

Lol, I am not setting out to reconstruct the cruiser -- all side decks were replaced just before the paint job two years back!

[starcruiser]

Oops, meant to say all CABIN sides were replaced....not side decks.

[Scout 30]

If this is just a cosmetic issue there are a couple of deck coating products out there that are reported to cover hairline cracks. I've read a lot about Kiwi Grip & it seems to work really well. I just heard about tuffcoat & it appears to be a similar rubberized roll on deck coating. If the decks are structurally sound & these are just gelcoat cracks due to age this would be the easiest solution.

KiwiGrip
Ultra Tuff and Tuff Coat Marine Non Skid Coatings

[topmast]

At the very best, the wood will flex more than a hard coat like polyester or epoxy. Bound to chek anywhere the bond between materials is exceeded. A web or fabric is nessesary on all seams.Just painting it on won't do much good. A complete sheath is best after preping the wood. There used to be an latex rubber (water based) it was originally steam pipe lagging compound ,went by the name of thorp's easy deck. Had 30 yrs of excellent use. Nonskid top coat with walnut shells.Now it's nearly all solvent based rubber but same need for fabric. Ask at a good deck and patio shop for whats available and trendy. But I'd stay away from hard plastics over wood.

[Snowpetrel]

Originally Posted by starcruiser

Thanks everyone for the helpful advice.
Unfortunately I am quite a long way from the boat but will take and post pictures when I next go there.

The plywood deck thickness I estimate at 1/2 inch maybe a little more. Beams under the largest area, the foredeck are at 1 ft centres and side decks are very narrow, maybe 8 inches but I didn't know what's underneath that area supporting nor whether the joints are scarfed --- but good concerns raised and a close inspection is warranted when I go up to see.

The cracks are not that many, and in random places on the foredeck and side decks.

Yes, on the anti-skid paint.

The cracks are very minor indeed...hairline, not necessarily straight, spidery. The sort of ones so narrow that sometimes we are tempted to paint over them hoping the paint will fill them.

Luckily there is no water inundation.

I suspect the cracks are the product of age more than anything else.

Lol, I am not setting out to reconstruct the cruiser -- all side decks were replaced just before the paint job two years back!

Sounds like the deck should be stiff enough. Its not heavyduty, but shouldn't flex enough to pop off glass?

I would just try sanding back to wood either side of the cracks, taper it slightly into the old glass and reglass these areas, overlapping the old glass.

If it keeps on cracking after this treatment then you will need more drastic measures in the future.

[DumnMad]

Sounds like drying/shrinkage cracks in the timber substrate. Maybe you are in a lower humidity area than before or you've recently installed a cabin heater ?

[starcruiser]

Originally Posted by Snowpetrel

I would just try sanding back to wood either side of the cracks, taper it slightly into the old glass and reglass these areas, overlapping the old glass.

If it keeps on cracking after this treatment then you will need more drastic measures in the future.

Snowpetrel, I would say that sounds like my inclination.

Originally Posted by DumnMad

Sounds like drying/shrinkage cracks in the timber substrate. Maybe you are in a lower humidity area than before or you've recently installed a cabin heater ?

DumnMad: I have a Planar diesel cabin heater in a big box at home awaiting installation. I started another thread on that project.

[Snowpetrel]

This is how I repaired the worst areas of my deck. I routered out a groove and inserted new plywood. Using the router and a guide it was easy to make straight uniform cuts.

I cut out strips of plywood to the same width as the router bit. Then it was easy to trim them to size, round over the ends and glue them in.

I just wanted to make 100% sure the lower plies of plywood hadn't got wet. My decks weren't glassed so the plywood checked in a few places. Hopefully your decks are fine and you can skip this step.