Moisture content of laminate?

I did a couple of searches and didn't come up with a prior thread, so I have a question.

What moisture content % is considered dry for a polyester glass-fibre laminate?

My boat is on the hard for the winter and the bottom paint stripped to the barrier coat.

The topsides read between 10 and 11% moisture.

The under water surface reads between 22 and 30% moisture.

The boat was built in 1975, as far as I know it was not built with fire-retardant resin.

The barrier coat is an unknown epoxy, applied in 1979.

Does the higher moisture content below the waterline indicate that the barrier coat has failed?

I'm going to monitor the moisture content over the next few weeks before doing any more work on the bottom.

At what point is the boat considered dry?

 

Polyester without a barrier will absorb about up to 35% of water; about the same as wood. The readings are rather high, but also depend on what meter you have. They don't all show the same. If the bottom paint is still on or the barrier coat has aluminum, copper or grafite filler, the meter will read much higher.

 

Thanks.

Using the same meter, the readings I have show a relative difference of 12-20% between above and below the topsides.

I'll take readings at the waterline edge of the barrier coat and the topsides. If the reading changes by 10% at the edge of the barrier coat, I'll assume that something in the barrier coat is adding a variation. I have no idea what might have been added to the epoxy barrier back in 1979.

The readings I have now show higher moisture as I move further down from the waterline.

I can't see how the moisture above the waterline should change much with the boat out of the water.

If I can't try for a target percentage (since meters read differently) should I expect the below waterline surface to dry out and reach the same level as the topsides?

 

Yes. One way to speed up the drying is with dehumidifiers. One inside. ON the outside, tape plastic from the waterline down and make sort of a bag. Put the other dehumidifier in it with a drain hose leading out.

 

Tja

along with the dehumidifier make surt that the building that your boat is in doesn't have any moisture coming in from outside the building. If the entire room is low in moisture it will in time draw the moisture out of the celcoat. Along with the dehumidafier an electric heater will speed up the process if you tent the boat. Since the bottom paint is off down to the moisture barrior and it is quite old from the sounds of it you might want to sand it and add two additional coats of epoxy barrior. Todays barriors are much better then those of years back. Hope this helps, Tom.

 

The boat is outside in rainy Vancouver. I've built a tarp cover to keep rain and snow off the deck. There's a poly tent from above the waterline to the ground.

The plan is to dry it out, repair blisters, fair the hull, 3-4 coats of Interlux barrier coat, and 4 coats of VC Offshore, sanded to 600-1200 grit then burnished with bronze wool.

It sounds like I should be happy if I get the moisture down to around 10% for all the surfaces before I start.

 

The hull will dry out a lot faster if you can strip that epoxy barrier coat off of it. Put a heater inside the hull. How bigga hurry you in?

 

The boat is out of the water until next spring, April or May.

I don't want to have to grind off the old barrier coat if it can be avoided. There are no major blisters, lots of 1/8 pimples. No delam that we've found.

As it stands now, the boat is very fair for it's age. Grinding off the barrier coat will just mean that much more fairing on the other end of the job.

If the moisture content doesn't drop fast enough for me to complete the job by my planned re-launch date, I'll have to consider the extra time and expense of removing the old barrier. Two dehumidifiers are a bunch cheaper than removing the old barrier!

 

Tja

If you use a random orbital sander to remove the old barrior you will have little to no finish work. Once it is dried out you can apply the new materials. If your going to apply the barrior coat and VC17 with rollers make sure you use foam rollers, both of the coatings will go on very smooth. Also don't use the white interlux barrior coat. The gray goes on much smoother. Good luck.

 

Unfortunately, the existing barrier-coat works both ways, much better to remove it before drying. The only truly permanent moisture barrier in my experience is to re-laminate with glass and epoxy. What we have found to work is to use three guys, three layers of 10oz cloth and a veil, WEST system epoxy with fast hardener, and last but not least, a Mexican radio station playing some good old oompah-oompah music.

Yoke.

 

...a good method to remove osmotic moisture in our lovely wet climate is to use a vacuum pump,,as in vacuum bagging.
You're at Reed Point?I'm based in vancouver,and happy to consult...604-716-7717