Moisture oozing from plywood

[JimJ]

I have just finished sanding back the topsides of my Hartley TS18. The reason for sanding was that the paint was peeling.

We have not had any rain to speek of here for about the last 4 months so the plywood appeared very dry after the paint removal.

On Saturday, we had a change come through and on Sunday high humidity, storms and rain. Anyone who watch Indy would know how much rain we had here.

The boat is undercover but large areas of moisture have appeared on the sanded plywood, mostly on the vertical sides of the cabin.

I taisted the moisture this morning and it is salty.

Questions are:

Is the moisture the result of salt water penetrating the paint and the moisture being attracted to the salt crystals in the plywood?

If it is salt, how do I get the salt out of the ply?

If I do nothing, what will the salt do to adhesion of the new paint?

Any ideas would be appreciated

Jim

[Dave Carnell]

The peeling paint and appearance of moisture make it sound like incipient rot. If it were mine, I'd give it a liberal painting with ethylene glycol auto antifreeze. This will kill any rot organisms and, in my observations of soft plywood spots will dry things up.

[thechemist]

The peeling paint shows that the paint is not sticking to the wood, and there are many reasons why paint eventually comes loose from wood. When you repaint, you might consider following Cleek's advice, resurrected in this thread, http://media5.hypernet.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=005833. This was also in another recovered thread, with more detail, http://media5.hypernet.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=005867.

As for ethylene glycol, much of the debate about that is in earlier forum archives lost [unless someone saved the Great Ethylene Glycol Firefights [many threads....there was no one by that name] but there *IS* this in current forum archives, Ethylene Glycol in Epoxy: A study of Insidious Degradation, at http://media5.hypernet.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=002241.

Before you paint any antifreeze on wood to be painted, even if not using epoxy, call your local manufacturer of the enamel paint or primer or CPES or whatever, and ask *THEM* if it is okay for antifreeze to be in the wood before you paint it, or if their chemists think that might interfere with paint adhesion.

[thechemist]

And one other thing....that moisture? It was most likely DEW, and tasted salty because there had been salt crystals in the air, or at some time the plywood had gotten wet with salt water which then dried. It is okay...just leave the surface in the sun and wind for a few days and the wood should dry out normally.

Contrary to folklore, ethylene glycol is not the cure for everything.

[JimJ]

Thanks for the replies so far.

There is no signs of rot in the plywood.

I figure that the waters of Moreton Bay have soaked into some areas of the plywood through the poor paint job. When the humidity in the air increased the salt cystrals in the ply attracted the moisture and presto - wet plywood.

As the humidity has dropped since Sunday the moisture has disappeared.

If I treat the plywood with CPES or Everdure and then paint what will happen to the salt cystals remaining in the ply?

I guess the CPES/Everdure/paint will slow down the absorption of the moisture vapour to where the salt crystals are, if it gets there at all.

From reading between the lines of the posts on the subject of rot, maybe a little salt in the ply may be a good thing!

[thechemist]

Any epoxy system I know of should cure perfectly well despite a bit of salt dissolved in the normal moisture of the wood.

As long as that moisture is hydrated to the cellulose, meaning the wood is below the fiber-saturation-point of roughly 25-30%, the salt should have no effect on epoxy-curing or cured properties.

Some epoxy systems themselves tolerate moisture better than others. If your clear epoxy on the wood dries or cures cloudy, that was excessive water in the wood picked up by the epoxy system, and which precipitated out as the liquid resins cured into a solid. That's not a good thing. Best to sand that stuff off, if it happens.