Hull Blister Repair

Introduction

The blisters were obvious as soon as the boat came out of the water.  They were everywhere, although definitely most pronounced at the stern.

When I was in the process of purchasing the boat I asked about the presence of bottom blisters, and was told that the boat has been barrier coated a few years ago.  I assumed that meant that the job had been done right and would no longer be an issue.  Unfortunately, that turned out not to be the case.

While I'm certain that some of the blisters are more recent, the majority turned out to not be gelcoat blisters.  They were in fact failed repairs of blisters.  Whatever was used to fill them the first time around had two problems.  First, the goop never set up properly.  It was as if the resin to hardener ratio wasn't mixed properly, or perhaps some kind of a caulking compound was used rather than a structural epoxy based filler.  Second, the goop never adhered to the hull.  I'm guessing surface prep had something to do with this.  Regardless of the failure's root cause, the symptom is tons of blister fillings lifting away from the hull.

The good news is that my hull isn't really compromised, it's just very ugly.  The bad news is, the repair is a pain in the neck.  The solution is to attach a 3/4" counter sink bit to a drill and grind out each hole.  As the pictures clearly show, this was no small feat.  When the main holes were opened up, there were a few spots that needed some fine tuning.  I used a dremel with a high speed cutter bit to clean up these few holes.

My initial research indicated that I would need to strip the bottom paint, and then apply a barrier coating to whatever I used to fill the holes.  This was really unappealing to me.  It would mean some very messy time behind a sander, or caustic stripping gel.  As I've come to say though, "it is what it is."  And I was resolved to move ahead.

Six10 As a Blister Filler

Right about the time I was researching which stripping gel to use I received a copy of Epoxy Works Magazine in the mail.  In it was an article that could not have been more timely.  The article describes a fast method for repairing blisters using West Six10 adhesive.  The unique element of this process is that the Six10 adhesive has an integral barrier coat which eliminates the need to strip the existing bottom paint or apply a separate barrier coat.  Not only would this be a tremendous time savings, but also a cost reduction.

The first problem I ran into was that it was nearly impossible to smooth out the Six/10 without slightly indenting the epoxy in the blister holes.  This meant that after a long session with the caulking gun I needed to sand, then go back and do it again.  After some time I did pick up a technique for slightly over-filling the holes for a one-pass fill.  Unfortunately, I lost a lot of time re-coating my first attempt.

The second problem I ran into was that despite the theory of this technique not introducing air bubbles, I ended up with many of them.  The third, and most substantial problem I ran into is that Six/10 is very tough to sand.  As a result, there were many places where the surrounding fiberglass was sanded below the repair area due to it being lower-density.  Now, this is really minor stuff you need to feel with a hand, or sight carefully in proper light to observe, but it happened.  Subsequent work I did using 3M Premium Marine Filler did not have this problem, and was MUCH easier to work with.  I would think that Interfill would be roughly the same as the 3m filler, and wouldn't hesitate to use either of them.  Lesson learned:  Six/10 is not a good choice for large-scale blister repair but it will work if you don't have anything else on hand.  Once the work is done, it's a very solid repair.

After what seemed like a lifetime of sanding the hull, I decided that this whole technique had been in vain.  I had sanded through the barrier coat in so many places that I was going to need a new barrier coat.  So, I began the process of sanding off the existing coat consistently.  This went reasonably quickly with a 8" random orbit sander from Home Depot (Rigid) and a big pack of 60 grit discs.  The areas under the hull were absolute hell to work on holding that heavy sander straight overhead.  But, eventually I finished and all was well with the world.

Keel-Hull Joint

Before I could apply the new barrier coat I needed to repair and fair the keel-hull joint.  It had been very sloppily filled with 3M 4200 sealant many years ago and was losing adhesion in addition to looking awful.  I was able to use a utility knife to cut out the bulk of the 4200, then used a Dremel sanding drum to grind out the remainder of the crack.  Finally, I ground out about a 6" area to make room for fiberglass tape.

After filling the crack flush with my remaining half-tube of Six/Ten, I used increasing width strips of 10 oz. fiberglass cloth to fill the ground out depression with three layers.  That ought to be plenty of strength for this application.  Final preparation was to lightly grind the edges of the glass job to reduce the amount of fairing needed.  The photo blow shows the keel after all that work.

Barrier Coat

Having repaired and fared everything, it is now time to apply a new barrier coat with Interprotect 2000e.  The estimation chart suggested 4 gallons to do the boat based on a typical 30-foot sailboat.  Mine being only 27 and shoal-draft, I knew the estimate was conservative.  Each gallon covered approximately 1-1/4 of my hull, which leaves me at roughly 5 coats.  Perfect!

Head-on after barrier coating:

Faired keel after barrier coating.  Compare this to the before shots!

The entire hull after finishing the job.  I couldn't be happier with it.

All in all, an expensive but very rewarding job.  I guess it was actually an inexpensive job when compared to having it done...  Everything is relative.

Fairing Blocks

Next on the to-do list was replacing the fairing block for the depth transducer.  The boat was configured by a prior owner to use fairing blocks on both the speed and depth transducers.  I have no idea why they felt it important to put a fairing block on the speed transducer, but none the less, there it was.

Yes, it is an un-sealed, cracked, and somewhat rotting piece of wood.

I used a 1/2" GRP sheet I purchased from McMaster-Carr to cut out three donuts which are about 1/4" larger diameter than the mushroom end of the depth transducer.  I scuffed and bonded them together to make a cylinder.  I then employed the original fairing block as a template to cut the angle on the cyllinder.  Finally, the new block was bonded to the hull using thickened epoxy finished off in a nice fillet.  This is much better than the wood blocks as it's not only stronger, but also water tight.

The speed transducer will not have a fairing block.  It will ride parallel to the hull as it should.  I'll just need to apply a minimal lay up to alter the angle it meets the hull.  Shouldn't be too hard.

Through-hulls

The through hulls came next.  The original parts enjoyed a (delaminating, rotting) plywood mounting block bedded in 3m 5200 adhesive, and a small nut to hold them in followed by an in-line ball or (in most cases) gate valve.  Not cool.  In fact, most of the through-hulls could be hand-wiggled as soon as the nut was removed.  I guess 3m 5200 isn't really forever.  The particularly scary picture below shows the 30 year old plastic fittings which stood between floating and sinking.  Scary.  Very scary.

I replaced all through-hulls with Groco flange adapters and in-line ball valves.  This is a very future-proof and solid system.  The sheer weight of these things immediately conveys they are built to withstand a direct hit from a super-tanker.  The flange-adapters are bolted through counter-sunk holes in the hull, and have 1/2" thick GRP mounting plates bedded and filleted in thickened epoxy.  Solid as a rock.

Anti-Fowling Paint

It seems that everyone in my marina uses VC-17.  I mean it, it looks like a copper-bottom club.  So, rather than fight local wisdom went with the flow and put on two coats of what everyone else uses.

Again, hindsight is 20/20.  I feel like while this paint works, it is restrictive.  Someday I hope to take my boat up the St. Lawrence, and down the coast of the US.  This paint will be useless in salt water, and to remove it, the whole thing needs to be sanded off due to its Teflon content.  You can't just put a top coat on to cover it up because nothing sticks to Teflon.  That's a nasty, time consuming, or expensive job.  I guess I'll cross that bridge when I come to it.

In the mean time, I will say that VC has a wonderful dry-time that makes it very fast and easy to get a coat applied, and it's really easy to paint the pad patches once the boat is in the sling, and lanuch 15 minutes later.  The other benefit is that you can be out of the water for a long time without the paint losing its effectiveness.  Some anti-fowling are very sensitive to this.  So, I could have done a whole lot worse.