Cockpit core repair

[sailortype]

I do have to respectfully disagree. One should never ever use polyester over epoxy. It does not adhere at the interface.
Epoxy is somewhat more adhesive in a secondary bond than polyester but on a deck job, polyester works just fine for decades. There is a bit of a learning curve when using it though if you are used to epoxy. You have to watch your mix ratios very carefully vis a vis ambient temps

 

[sailortype]

further on the comment about fairing compound... a lot of people will use marine Bondo. This apparently contains a large amount of talc and if not careful and if not covered very soon, it will absorb moisture and negatively affect eventual finishing

 

[dlochner]

I do have to respectfully disagree. One should never ever use polyester over epoxy. It does not adhere at the interface.
Epoxy is somewhat more adhesive in a secondary bond than polyester but on a deck job, polyester works just fine for decades. There is a bit of a learning curve when using it though if you are used to epoxy. You have to watch your mix ratios very carefully vis a vis ambient temps

When epoxy cures it will leave an amine blush. This is a water soluble wax like coating. It will interfere with anything bonding to the underlying surface. Sanding only further embeds the blush. If the blush is washed off with water and a green scrubby polyester will adhere. Take a look at these videos:

If you dig around on the West System website in the Epoxyworks archives, you'll find an article based on these videos.

Preparation is important. If the surface is not properly prepared, it does not matter what product you use, it will fail.

 

[sailortype]

the Boatworks videos are not science but anecdotal. I have never really seen any scientific empitical evidence on this issue and there are as many opinions as there are people discussing it. Epoxy has an initial cure and a final cure which takes a significant amount of time to achieve; there may be traces of blush generated during this time which could adversely affect the interface bond. I think it comes down to one of thos 'your boat, your choice' things. It is just not something I would ever do, put polyester over epoxy and especially in a critical application. And there really is no need to.... polyester is very tough stuff and more than adequate for most applications.
And I say again... I have always been an epoxy proponent... but in the last couple of years doing structural work and some parts reclaiming off older boats, it becomes apparent very quickly how tenacious polyester resin/glass really is. One hears 'poor secondary bond', one hears 'does not adhere to wood'. The bulkeads in 40-50 year old boats are weakened due to water intrusion from above; the actual bond of the polyester glass to the hull (secondary bond) and the bulkhead itself (plywood) is completely intact and you need some very serious tools and effort to remove them if replacing.

 

[dlochner]

the Boatworks videos are not science but anecdotal. I have never really seen any scientific empitical evidence on this issue and there are as many opinions as there are people discussing it. Epoxy has an initial cure and a final cure which takes a significant amount of time to achieve; there may be traces of blush generated during this time which could adversely affect the interface bond. I think it comes down to one of thos 'your boat, your choice' things. It is just not something I would ever do, put polyester over epoxy and especially in a critical application. And there really is no need to.... polyester is very tough stuff and more than adequate for most applications.
And I say again... I have always been an epoxy proponent... but in the last couple of years doing structural work and some parts reclaiming off older boats, it becomes apparent very quickly how tenacious polyester resin/glass really is. One hears 'poor secondary bond', one hears 'does not adhere to wood'. The bulkeads in 40-50 year old boats are weakened due to water intrusion from above; the actual bond of the polyester glass to the hull (secondary bond) and the bulkhead itself (plywood) is completely intact and you need some very serious tools and effort to remove them if replacing.

Granted, West System has a vested interest in promoting their products, however, here are a couple of articles from their website that are more data based.

https://epoxyworks.com/index.php/polyester-over-epoxy/
https://epoxyworks.com/index.php/west-system-epoxy-vs-polyester-for-fiberglass-boat-repair/