Polyester under a last layer of epoxy?

[Runner]

On small boats like those I might make here at home, how weak would using polyester resin until the final layer of glass and then going to epoxy be in comparison to using epoxy all the way thru the layup?
The cost savings is considerable for a 16 foot boat.

[Ziggy]

Most boats are constructed entirely using polyester (vinylester) resin. Are you considering epoxy as a barrier against osmotic blisters?

[Runner]

Yes, but not just for that. It is supposed to make a stronger more resistant to damage surface also.

[Boracay]

Have you considered using generic epoxy resin?

The stuff I use is about $A15 per litre, that's about $10/litre or $40/gal. in your expensive American dollars.

I would expect that good polyester resin would be close to that in price.

My memories are from a while ago now, but cheap polyester can cause a heap of problems. Cheap epoxy mixed in the right ratio and at the right temperature is reliable and relatively easy to use.

[Ziggy]

If you're concerned about resistance to abrasion, punctures, tensile strength, or stiffness, I think the choice of resin may be less important than the choice of substrate (e.g., kevlar). But high tech substrates are expensive also. I'm not positive, but I don't think that a single layer of glass reinforced epoxy over many layers of glass reinforced polyester would make that much difference in strength.

I think the advantages of epoxy are primarily in being less susceptible to osmotic blistering, and also in making stronger repairs (i.e., it's an excellent adhesive).

[jim lee]

Try vinyl ester instead of poly. Does the osmotic sealing, is stronger than poly but works well with it. And, you can put on a nice tough gelcoat finish that epoxy doesn't allow.

-jim lee

[Ziggy]

I second Jim Lee's suggestion.

[westsail42]

Originally Posted by Runner

On small boats like those I might make here at home, how weak would using polyester resin until the final layer of glass and then going to epoxy be in comparison to using epoxy all the way thru the layup?
The cost savings is considerable for a 16 foot boat.

Of course, layering epoxy over polyester will have to be a secondary bond. Secondary bonds will always be weaker than the primary (chemical) bond you would get with polyester-over-polyester. Any strength gains from such an expoy suspended laminate may be offset by the weakend secondary bond between the last two layers. For a small boat like 16 feet, this may not be a big issue, but considering this method for "strength" reasons is misguided (my opinion). As for osmotic blistering protection, an expoy barrier coat would provide the same.

[Runner]

Thanks guys!
My understanding was that the epoxy bond to polyester was superior to the polyesters bond to itself, and that nothing was given up in that part of the trade off. The superior water resistance/barrier was my main concern since I will be using wood.
I ordered the Sailmakers Apprentice today. Guess I will order the Boatmakers Apprentice next week!

[westsail42]

Originally Posted by Runner

Thanks guys!
My understanding was that the epoxy bond to polyester was superior to the polyesters bond to itself, and that nothing was given up in that part of the trade off. The superior water resistance/barrier was my main concern since I will be using wood.
I ordered the Sailmakers Apprentice today. Guess I will order the Boatmakers Apprentice next week!

Depends on what type of bond you are talking about.

A PRIMARY bond between polyester/vynilester and epoxy does not work because they are two different chemical systems.

Polyester-on-Polyester and Polyester-on-Vynilester are strong as PRIMARY bonds.

Polyester-on-Polyester as a SECONDARY bond is weak because Polyester shrinks slightly when it cures.

Vinylester and Epoxy does not shrink and is therefore strong as a SECONDARY bond (on any polyester, vinylester and epoxy).

[jim lee]

How is the west sail going? From the outside it looked like the project was stalled. From your blog I see you been busy!

-jim lee

[westsail42]

Originally Posted by jim lee

How is the west sail going? From the outside it looked like the project was stalled. From your blog I see you been busy!

-jim lee

Hey Jim,

Yeah, things are pretty much unchanged on the outside (though we did get the rudder completed and installed). But, a lot of progress has been made on the inside this past year (mostly rough-in work). That will start changing next year as I hope to get the hull and deck bonded. Around then some deck hardware will start to "appear".

-rob