Plywood and polyester

Although I've used plywood sheathed with epoxy/glassfibre for years on all sorts of craft from canoes to 50ft Dutch Barges, the ever-rising cost of epoxy is starting to make me think the unthinkable. With the same sort of reluctance that Ignatius Loyola might have felt if contemplating conversion to Lutherism, I'm now wondering whether a successful method has been developed which would enable me to switch to polyester. I gather that a certain level of success has been achieved with G4 and similar polyurethane primer/tie coats. But does anyone know if the game has moved on at all from there?

Edmund

 

My use is small and name brands (West, System Three) are worth their price for a quality guarantee and consistency.

Epoxy is not where I would cut corners.

Before switching to polyester over plywood, a generic industrial epoxy formulator could be evaluated.

I am not knowledgeable on UK pricing or suppliers.

 

Thanks for your reply. I absolutely take your point. I am v. reluctant to switch from epoxy and would not consider doing so for joints or any 'structural' or 'secondary' bonding. But, even non-West System epoxy in the UK now costs the thick end of £10/kg ($7/lb), 4 to 5 times the price of polyester.

Many of the boats I build are used on the inland waterways, so abrasion resistance is important. Otherwise, I would consider switching to a high-end marine ply and doing without sheathing altogether.

Fortunately, since my hulls are often built from multiple layers of 4mm ply bonded together under vacuum with staggered joints, it would only be the outer skin that had to be up-graded. On the other hand, the higher the quality of the ply, the oilier the wood tends to get and the more difficult adhesion becomes. Also, a lot of my designs are used by DIY builders who don't have facilities for vacuum-bagging.

I could try and devise my own method of using polyester/glassfibre for sheathing, but I am reminded of the phrase 'it may be the early bird that gets the worm, but it's the second mouse that gets the cheese.'

So I'm keen to find out what others are doing.

Peter

 

Dont know much about polyester bonding. Epoxy is king these days.


In the old days of polyester over ply, decks for instance, I used to see builders sand and prepare the ply... then wash the ply with clean fresh water and a soft brush... dry...then prime the ply with resin. The thought was that the water raised the grain of the ply and gave the polyester a better mechanical bond. I dont know if this method has stood the test of time.

. You should direct your question at the resin producers .

 

Both Joel at Bateau.com and Paul at epoxyproducts.com ship overseas and their epoxy is 1/2 to 1/3 that of West System.

Polyester can be made to work on plywood, but to be effectively waterproof, you need enough thickness, that any savings over epoxy is moot and you'll still have the inferior peel strength.

 

A thought from "out there."

Seeing as how we're bagging more items and this method usually squeezes out voids and creates some clamping pressure what about ONLY in between the plys going "old tech" and using resorcinol glue?

Cheap, strong, waterproof, terrible with gap filling calling for close tolerance joints. Slower set than epoxy or polyester but just leave the vacuum on?

Then just use epoxy on the outside.

I'm not recommending this, just throwing it out there after my first sip of coffee

Steve

 

What are you thinking of gluing with the resorcinol - the fabric?

 

it has been my experience, and that of my customers over the past 20 or so years that polyester resin disbonds from wood after 5-10 years. That is why fiberglass boats are 100% fiberglass and not a blend of wood and polyester resin.

paul

 

Yes, it is very common to find delamination of polyester FRP skins from plywood bulkheads after 10 years in service.

I have not seen any study of the effects of loss of adhesion of thicker polyester over wood elements - stringers or transoms. These are commonly replaced after 10-20 years due to rot anyway.

 

I have seen test data, of the physical properties of polyester, on various substrates and it's not very good, particularly compared to epoxy. It has a permeability issue, so it needs to be thick, which bring on it's own issues.