Prep Before Barrier Coat

[pvanv]

I have stripped the 40-year-old , flaking bottom paint from our 28' sailboat, and will be applying Interlux 3000E and 2000E barrier coat before painting with a good ablative bottom paint. This is not a blister repair -- there aren't any blisters, and the boat lives in Lake Ontario fresh water, so I'm not expecting any future blistering. Yet, after going through the labor to strip the bottom, a barrier coat seems prudent. The stripping was done with Soy Strip, and the removal of the stripper/paint sludge combo was done with a 3600 psi pressure washer. The hull is now very clean, the boat is canvas-covered outdoors in western NY, with excellent ventilation, and the laminate should be very dry in a few months.

I have discovered thousands of spider cracks in the gelcoat. These are not point-load stress related, as they are pretty much everywhere. Of course, there are more cracks in high stress and high flex areas. Since the old-technology, hand-laid, solid glass hull is of the era when the gelcoat was way thicker than needed, it seems that this spider cracking is due to the thick gelcoat not flexing well enough. There doesn't seem to be any structural issues with the 1968 Hinterhoeller hull. Laminate moisture is nil.

While long boarding and fairing the hull, I plan to sand a the gelcoat thinner, which hopefully will reduce future spider cracking. I am wondering whether I must vee each crack open and fill with Interlux Watertite (or equivalent), or if I will get sufficient penetration and edge-bonding of the gelcoat cracks if I use Smith and Company CPES. I'd rather not strip the hull again in a couple of years because of fresh cracking, but the labor to vee each gelcoat crack might run into hundreds of hours.

Thoughts?

 

[ondarvr]

Re: Prep Before Barrier Coat

That type of cracking is a nightmare, there's just no easy way to make sure they don't come back without removing them completely. If the hull is flexing much they'll most likely migrate through the epoxy to the surface if you don't grind them out, but sometimes you get lucky.

Sand out a few and see if they're only gel coat deep, if they go deeper more work will be needed.

Not a good answer is it.

 

[drewpster]

Re: Prep Before Barrier Coat

If the spiders are only gel deep and proliferate over the entire hull, I wonder if the gel needs to be completely peeled? Maybe with one of those fancy hand planers?

 

[pvanv]

Re: Prep Before Barrier Coat

Yes, the spiders are only in the gelcoat, not into the laminate. These boats were laid up in the early days of fiberglass, and the builders really did not know how thick to make things -- new technology and all, so they erred on the heavy side. The gelcoat is more than 1/16" thick in most places, unlike modern lay-up schedules, making it brittle. Likewise, the laminate near the keel stub is well over a half inch thick. Again, typical of 1960's Hinterhoeller glasswork.

The project might never have come up, except that the really old bottom paint, having lost its adhesion to the gelcoat after 40 years, would occasionally shed, taking my new, expensive bottom paint with it. After several years of feathering and repainting, I finally bit the bullet and stripped all the bottom paint. Since I already have the stripping labor invested, a barrier coat seems prudent. Some have suggested just applying the barrier coat to the bottom as-is, while others have insisted that each spider needs to be v-ground to the laminate and epoxy-filled before doing the barrier coat. I'm not convinced that the new epoxy filler might not crack as well -- possibly where it joins the old gelcoat. Of course, there will be the added film strength of the barrier coat, which should help some, encapsulating and bridging the spiders.

The spiders are (structurally) pretty insignificant -- where they appear in the topsides areas, I have just waxed them, and no issues. They are almost invisible. The real question is whether I will eventually get water migration into the laminate -- and possible blistering -- below the waterline. This is a 7,000-pound, 28-foot sailboat (with external lead ballast), and does not flex a lot, but it does flex some, and that seems to be the cause of the spidering.

No matter what, the process of long boarding and fairing will definitely thin out the gelcoat, increasing its flexibility, and may prevent future spidering. Since I'm in western NY (Buffalo), working outdoors, and the launch date is non-negotiable, scheduled for the end of April, it's doubtful that I can get the weather and time for a gelcoat peel, though I do have a peeler available. So it comes down to whether I can get away with edge-bonding the spiders with several coats of penetrating epoxy before applying the barrier coat (saving a couple of weeks of cold-weather labor), or whether that will allow the spiders to continue to open up.

Thoughts?