Deck Sheathing: Alternatives to Fiberglass Fabric and Plastic Resins

What next?

Rain on the flowers,

Rain on the trees,

Rain on the rooftops,

But not on me!

 -- Mother Goose

Deck Sheathing: Alternatives to Fiberglass Fabric and Plastic Resins

Plywood decks are strong and easy to build, but not naturally waterproof. Sealers, paint and/or saturating resins help, but are relatively fragile... wear and tear from traffic soon allows water to penetrate. Sheathing is an attractive option.

Sheathing is a layer of fabric which is saturated by - and provides a matrix for - a waterproof substance which, once cured, bonds this layer to plywood, forming a composite structure.

Nowadays, a matrix of fiberglass cloth set in epoxy or polyester resin is nearly universal. For many reasons, Anke and I prefer an alternative suite of materials and techniques, which we learned about back in the day. To my mind, they are much more amenable to amateur construction, and worth considering in many applications around a home-built or modified vessel.

Current Practice

GRP (Glass Reenforced Plastic aka fiberglass) is undeniably wonderful stuff. It can take on most any shape, and be layered up into a skin which is itself structurally adequate for hull construction. But for sheathing its strength is wasted.

Pros are that the matrix is reliably waterproof and generally long-lived. There is abundant information available for its application. These resins are very versatile, and may already be in service for other tasks around the boat, spreading the costs of special tools and accessories.

Cons...

Fiberglass isn't particularly nice to work with. It tends to fray messily, and when trimmed, it often produces needley, itchy li'l hairs. Both are made worse when smeared with uncured resin. No fabric likes to make a sharp turn, but glass fabric is particularly fussy. Grinding to smooth is toxic and itchy.

The relatively hard matrix can craze or shatter under impacts, leading to delamination and requiring a wider area repair. This is of particular concern for bottom sheathing where grounding the boat is desired. Even small rocks can compromise GRP sheathing, so substantial repairs can be a seasonal chore.

Worst, glass is inelastic and plastic resins little more so... plywood expands and contracts a bit with humidity and temperature, and glass doesn't like to move with it. The result is shear forces between wood and sheathing, increasing chances of delamination. White finishes help considerably to reduce thermal movement.

Note: In this regard epoxy, which has smaller molecules, is preferable to long-chain polyester resin, and its performance is generally considered to be acceptable. Good results have also been obtained with polyester resin with good attention to surface preparation, catalyst proportions and thermal sequence). In other words, the problem aren't insurmountable in either case.

Plastic resins are toxic, exothermic chemical admixtures (can spontaneously burst into flame) involving solvents; they do best in fairly well controlled building environments and impose strict working times; they're expensive and require special tools and heaps of disposable accessories.

Acrylic Fabric, an Alternative to Glass

Acrylic fabric – often referred to as DYNEL after a former, proprietary product of Dow Corporation – is an inexpensive, soft yarn synthetic (rot proof) which wets out well, conforms well, frays only moderately, and is both elastic and highly abrasion resistant. It is often used for drag strips to protect the keels of GRP kayaks from chafe.

Acrylic has been extensively used with epoxy resin to sheath high-end plywood hulls by Reuel Parker and others. It does not contribute significantly to total strength, but that is supplied by the wood it protects (note that dry wood is much stronger than wet).

Being elastic, it moves with plywood. Crazing or shattering, when it happens at all, is limited to a local area. Shear stresses are low, so local damage is much less likely to spread, making repairs a much smaller task.

Grinding acrylic doesn't throw a cloud of glass shards, but does tend to fuzz and pill. This can be shaved, however, and if necessary, a final topcoat of pucky may be applied after grinding.

One caveat... I've read that acrylic should not be used with polyester resin. Dunno why. Our samples showed no short term problems with polyester resin, catalysts or cured resin (that is, nothing melted). In the longer term, we used acrylic/polyester for our dory chine chafe strips... they held up as well as their glass epoxy predecessors despite much poorer application conditions.

Non-PlasticResin Pucky

For many years, DIY boat builders decked with various fabrics (even burlap!) set in asphalt tar, paint and (of particular interest) lagging compounds – various latex puckies used to saturate cloth wrapping for hot water pipes. The huge advantages are economy, ease of application and water clean-up. It is also very easy to repair.

One favored product was ARABOL (now unavailable), which was essentially waterproof Elmer's Glue (both made by Borden Corporation as lagging compound). We used this with glass fabric to sheath LUNA's decks, which were still looking good at the 18 year mark. We know of one lapstrake(!) hull which was fully(!!) albeit sloppily sheathed in ARABOL/burlap to give it an extra decade of live-aboard life.

Lagging compounds tend to finish rather soft... a thumbnail can leave a dent, and a floppy block can wear a hole. Chafe protection in choice spots and prompt repairs easily keep up. 

Overall adhesion, while sufficient, is rather low. This can be an advantage when replacing as the entire matrix, once started, can be peeled away with relative ease. This makes it a good underlayer for asphalt products without commiting the underlying wood.

In WAYWARD, Anke and I are experimentally decking with acrylic plus TiteBond III. It cures to a plastic, slightly elastic sheet which is harder than lagging compound and fairly similar to epoxy/polyester. It is cheap (in bulk), very low toxicity, needn't be mixed and wears a lot of hats.

This method passed our 7 month immersion tests in salt water. It adheres to our wood (radiata pine)'s full fiber strength, so is very strongly bonded.

At present, WAYWARD's sheathed and topcoated deck has been sitting out in winter rain/freeze/thaw cycles for four months, and reports are that it's "looking good".

In a sense, we were fixing what weren't broke; we were perfectly happy with the soft deck. But TBIII certainly wears more hats than lagging compound, so we decided to take the risk for science.

You're welcome!  8)

Application of Fabric plus Water-Based Pucky

Application consists of the following steps:

1. Sweep and wet-tack for a dust-free deck
2. Lay out fabric (may be overlapped, but abutting is sufficient and smoother)
3. Wet out fabric (Yep. Water... drippy wet)
4. Paint on pucky (may thin somewhat with water, if necessary for low-drag)
5. Dry
6. If not satisfied, repeat from 3 (a bit of weave left provides texture)
7. Prime and top-coat

The first round of 1-4 is the primary adhesion step. As the water dries out of the weave, waterbourne pucky wicks (is drawn by capillary action) down into pores of the plywood substrate, creating a permeating bond interface. Subsequent layers build to coat and fill the weave.

On a warm, dry day, water evaporates quickly. If the fabric is drying ahead of you, consider keeping a water-brush on hand to refresh the wet. Without that water, wicking is reduced, and glue may not dilute and penetrate the fabric or wood surfaces for full adhesion.

At the end of the first pass, the fabric is only lightly bonded, however, and can be fairly easily torn away. It is reenforced by subsequent passes, however, and the result is firmly attached.

Consider whether to leave some weave for texture (thin matrix), or fill past the top of the weave for longevity (thick matrix). In the latter case, you might consider added texture in the topcoat.

My only semi-eddicated opinion is that green (not completely cured) layers bond better. Thus many layers can be applied in a single day. I especially like to prime over a green layer, in effect gluing the primer to the matrix. The whole seems to cure well over ensuing days (possibly even faster than the generally indicated 24 to 48 hours).

Topcoats

Most standard topcoats can be used, keeping in mind that the more elastic the matrix pucky used, the more elastic should be the topcoat.

Our preference is for flat latex housepaints (trim or porch enamels are most durable). If we leave the weave unfilled, this provides sufficient texture for good footing. If desired, SKID-NO-MORE adds ground rubber to a latex base.

Aluminumized trailer paint (ATCO SILVER SEAL is the best we've tried), applied with a short nap roller, provides great footing, and can be built up to an independently waterproof layer, especially along interior corners. It's messy, however, with lots of solvent... we've abandoned it for the most part.

Repairs

Puncture – Repair any wood damage. Patch with fabric, if necessary. Rebond as above. For small punctures, a shot of latex caulk or dribble of latex paint will do.

Abrasion – Patch, bond and topcoat. Consider a chafe patch or ropework, or eliminate the source.

Delamination – Slice the matrix to peel back. Inject latex caulk as deeply as possible. Work away from the slice to fill the blind pockets, then reverse and work toward the opening. Squeeze any excess out and smooth. Topcoat.

Repaint – We like to spot paint as the topcoat wears through to primer, and repaint fully when it gets widespread. On SLACKTIDE, that seems to be a full recoat every four or five years. We've never tried it, but I see no reason not to add pucky over paint if general re-thickening of the matrix seems advisable. In general, we try to avoid wearing down into the fabric itself... so far, no more than paint has seemed necessary.

* * * * *

Acrylic is great stuff, and useful in many places where glass fabric is structurally unnecessary. It happily works with epoxies, but also with much friendlier, water-based products.

Consider the application and what is demanded of it. Will a cheaper, easier, more benign suite of materials do the trick?

If so, why not?

A Note on Longevity

Rule of thumb on fishboats was that a lagging/burlap deck would last about 15 years. Synthetic fabrics were thought to last longer somewhat longer, perhaps to 20 years. Use and quality of application likely played a role.

Epoxy decks are generally considered to last 20 to 25 years, I've read.

The problem for comparison had been that most side-by-side data was generated by yachts vs non-epoxy workboats (which have only fairly recently 'gone over'). Even full-time cruisers don't give their decks the workouts that a fishboat takes. In addition, a mid-season epoxy repair is a non-starter for a working vessel. That being said, non-epoxy fishboat decks likely gets/got more new layers slathered on, here and there, than the yacht.

My guess is that they may be fairly comparable if traffic loads are taken into account?