Polyester on exterior ply

[whoosh]

Originally Posted by Jimbo1490

There are several reasons why polyester resin doesn't stick all that well and epoxy does. There's the elastic modulus as Par mentioned. There are flexibilizing agents that might be used to alleviate this problem, but like any resin modifier, you gain in one area but lose somewhere else, tensile strength this time. And poly resin does not have a whole lot of tensile strength to begin with, so this is not considered a good trade-off which is why it's not done.

The next problem that plagues poly resin's adhesive properties is that it always shrinks as it cures. All resins shrink some during cure, but poly shrinks A LOT, like 3% or more in some cases. So it actually retreats from close contact with the surface that you are trying to bond to. This is not good. Epoxies typically shrink .1-.3%

The third problem has to do with surface tension. Surface tension in a liquid prevents 'wetting'. Water is wet for instance, but it does have some surface tension. When you put soap in the water, it kills the surface tension and now the water is 'wetter' and adheres to things better, capillaries better, spreads into a thin film, etc. Oils have about *zero* surface tension and wet and cling to surfaces very well.

Well it's the same with poly and epoxy. Epoxy is just 'wetter' than poly and seeks out all the little micro cracks, craters and divots in the surface down to the molecular level which gives you way better adhesion.

And with SO MANY different curing agents and modifiers (and much higher native tensile strength to work with) you can tailor the cured state properties of epoxy resin to match the elongation, flexibility, etc of wood quite easily so the bonds will be so much more permanent.

Don't waste you time with poly!



Jimbo

Jimbo, that issa nice post, thanks, , but what is wrong with , as on does on first up coats varnish, thinning the poxy with thinners,, got no choice but to sink in then has it, same with poly? thin down 50 % with first coat, sand back and then apply more
i am a man of metal, but very soon will do a project with superstructures of ply, glassed over, so thisis off interest to me

[sproxxin]

Thanks Manie B

I will be building the "cheap canoe". I could only source some exterior ply at this time of the year. I also decided to "bite the bullet" and go with epoxy.

I will be taking pics and hope to upload to show progress of construction...

[Wynand N]

them again, after building the plug of my 6m day sailor with plywood, we gave it a liberal coat of polyester resin (no sanding or other preparation done), let it dry and the laid a fibreglass cloth over before starting the fairing drill.
Stuck to the ply like there is no tomorrow

[sproxxin]

I was wondering if its possible to spray FLOWCOAT on top of epoxy?