Gelcoat (polyester) over Epoxy; A Synopsis

Most production fiberglass boats are made with polyester resin.  Epoxy is a wonderful material for repairing polyester fiberglass boats. One reason for this is the ability of epoxy to form a stronger mechanical bond to a damaged laminate than polyester resin. Epoxy also provides a better moisture barrier than polyester resin.

Our customers' fiberglass repairs range from filling a minor screw hole to completely rebuilding the bottom of a boat that ran aground. In most cases, a final finish needs to be applied over the repair. If the repair is above the waterline, high-gloss paint or gelcoat are often used to reproduce the original appearance. If the repair is below the waterline, antifouling paint is most often applied over the repaired area. However, customers often want to restore the original gelcoat finish to below-waterline repairs where the boat is kept in the water and no antifouling paint is applied. These customers are concerned about the gelcoat's long term ability to adhere to the area repaired with epoxy. There are good reasons for this concern, but if you understand the materials involved and follow recommended repair procedures, you can successfully apply a gelcoat finish to a boat bottom repaired with  epoxy.

In repairs above the waterline, gelcoat applied over properly prepared  epoxy has a great track record. Many boats have been repaired this way and are performing well in a variety of climates. Boats that are dry sailed or trailered have also had good success with gelcoats on underwater epoxy repairs. Concern about adhesion occurs in situations where boats are in the water constantly and need to have a high level of finish on their bottoms that requires gelcoat. Such a situation might occur with a boat kept in a pristine lake where antifouling paint is often not used. Since the polyester gelcoat is not very good at preventing moisture from permeating through to the laminate, there has been a concern that, over time, the gelcoat will detach from the more moisture resistant epoxy layer.

Understanding the materials
Polyester resin laminates achieve their highest strength when the bonds between layers of fiberglass are chemical or primary bonds as opposed to mechanical or secondary bonds. The manufacturing process for polyester boats has been developed so that a chemical bond exists between the gelcoat and the laminate. When a polyester boat is built, polyester gelcoat is first sprayed onto the mold surface. The first layer of the laminate is then applied to gelcoat, which has not completely cured. The two layers eventually cure together with a chemical bond between them.

Applying gelcoat to a cured laminate relies on a mechanical bond. Because of the difference in curing chemistry, it is not possible to achieve a chemical bond between epoxy and polyester gelcoat. We developed some tests, to determine whether or not the mechanical bonds achieved between gelcoat and properly prepared, cured epoxy were strong enough to achieve a durable repair.

Testing
The test program we developed was designed to determine the effect of moisture on the adhesion of polyester gelcoat applied over fully cured epoxy. The tests were done on a gelcoat finish over a simulated repair made with epoxy. The test included a variety of finishing processes. In addition, we varied the length of time the epoxy was allowed to cure before being subjected to high moisture conditions. For comparison, we also tested gelcoat adhesion to a polyester laminate using West Marine Boaters Resin™.

Two popular gelcoats were used, Minicraft™ and Evercoat™. Interestingly, Evercoat states on their can that it is not to be used over epoxy. Although we often suggest following the manufacturer's instructions, we believe, in this particular case, the manufacturer is overly conservative. As with many products, the surface must be prepared properly; if it is not, then poor curing and adhesion may result. Polyester materials can be affected by amines in the epoxy hardener. If the hardener has not fully reacted with the epoxy resin or the amine blush is not removed from the cured surface, problems can occur. Proper surface preparation will prevent these problems.

Preparing the test panels
The test panels were laminated from three layers of 738 Biaxial Fabric (15 oz biaxial with .8 oz mat) wet out with 105 Resin and 205 Hardener. Panels were covered with 879 Release Fabric and allowed to cure. This laminate provided a stiff panel that simulated a patch used to repair the bottom of a fiberglass boat. After the Release Fabric was removed, a variety of finishing processes were applied to the panels. These included using 742 Glass Fabric (6 oz), 407 Low-Density Filler, and 410 Microlight™. All of the test panels had a sealer coat of 105 Resin/206 Slow Hardener to fill the weave and seal the scratches in the sanded fairing compounds. Slow Hardener was chosen because its slower reaction time would be more likely than 205 Fast Hardener to reveal any adhesion problems.

The gelcoat was applied using a pressure pot type spray gun. It was thinned using 10% Acetone by weight catalyzed at 1% with MEKP (Methyl Ethyl Ketone Peroxide). After the gelcoat cured for two weeks, PATTI (Pneumatic Adhesive Tension Testing Instrument) studs were applied to the surface.

One set of PATTI studs were pulled for a baseline value. Panels were then placed in our 100% humidity/100°F environment room. The conditions in this room accelerate any effect that moisture will have on the laminate. The remaining PATTI studs were pulled at three, six and nine weeks to determine the long-term effect of moisture on the bond.

Figure 1 (right) lists the different material combinations that were sprayed with gelcoat. The graph for each combination shows the tensile load required to pull the stud off the panel at four time intervals, illustrating the trend of the bond strength when the gelcoat is applied after allowing the laminate to cure for 36 hours and 7 days. Each data point represents the average of five stud pulls.

Conclusion
The overall effect of moisture on the gelcoat's bond to both the polyester and epoxy panels was negligible. The graphs show very little change in the adhesive strength, and much of the variance is within the tolerance of the test method. When the studs were pulled, the failures occurred in the fairing compound, cohesive failure of the gelcoat, or in the bond between the gelcoat and laminate. Since these three modes of failure occurred under similar loads, it indicates that the strength of the bond is close to the cohesive strength of both the gelcoat and the fairing compound. The nine-week exposure may not simulate what could happen after many years, but it does indicate that the gelcoat bond to epoxy laminates is a good bond and should perform well.

Our test clearly demonstrates that polyester gelcoat will bond to a properly prepared  epoxy as well as to a cured polyester laminate. This test confirms that polyester gelcoats can be applied over cured  epoxy on repairs below the waterline.

 

DIY Boat Projects Magazine and web site

I thought I would bring this excellent magazine and web site to my readers attention. They have many useful boat projects for the DIYer and can be found here.
Looks like the DIY boat projects for this mag has turned into a Forum type of online help. You can still link back to it; but it is not the same as when I first came across it.
However, try this site for questions and answers. Done by the Sail magazine 

How To Make Soft Shackles

How To Remove Wood Bungs to Expose Screws

Windsong shows us how to remove wood bungs to access the screws. Here is his method...
As can be seen, the bungs aren’t exactly easy to locate.  Sometimes you really have to look close with good lighting to find them all.  Before you try to remove any piece of wood, triple check that you have located all of the bungs.  If you try to pull off a piece and a screw remains, you have a good chance of damaging the wood.  Ask me how I know

Once you have located the bungs, you need to assemble your tools:

•  An impact driver with the correct bit (flat head for my screws)
•  A drill with a 1/4″ forstner bit
• Hammer
• Cheap 1/4″ chisel

By cheap chisel, I refer to the kind you can get from Home Depot in a 5 pack for about $15. I am not referring to a fine woodworking chisel. Some woodworking tool enthusiasts see my use of chisels as an improvisational tool and nearly choke on their drink at the misuse of the tool. These people need to chill out a little bit and realize that the chisels I use are very cheap, sturdy, and can take abuse. Save the fine woodworking chisels for their purpose, but don’t be afraid to abuse the cheap ones for whatever you can come up with.

 The first step is to drill out the bung in the center using the forstner bit.  Use a forstner bit that is smaller than the bung size, in my case a 1/4″ bit.  Apply enough pressure and drill until you reach the screw hidden beneath the bung.  Do not drill too hard into the screw or you can tear up the head.

 Grab your chisel and insert it into the small hole you made.  Work the tip of the chisel around in the hole, applying pressure and spinning it around.  The goal is to expose the screw head and slot like so:

Most of the time the screw slot will be filled with glue or wood from the bung, and the chisel is required to clear the screw slot.  Usually you can do this with a little effort by hand, but a tough one will require you to hammer the chisel into the slot to clear the junk.
This brings up a point about these slotted screws.  Before I developed this system, I began to seriously loathe slot headed screws and cursed the boat maker for using them to hold on everything.  The slots are so easy to slip out of (until I found the joy of the impact driver), and easy to strip the slot of its edge.  I figured anything other than a slot headed screw would be preferred.
However, the genius of the slot headed screws is that if you strip the head, you can still use it with some working of the chisel and hammer.  Insert the chisel into the slot and hammer away on it, using both sides of the chisel to create a deep enough groove for the driver bit to latch onto.  If these were phillips or other type of screw heads, you would strip the head and be completely screwed (lol) and would have to resort to more difficult methods of removing the screw.  The slotted screws have enough un-cut space on the head to give you room to re-bore the screw slot and give it a new edge.   While some screws took a bit of effort with the chisel and hammer, re-stripping the screw, chisel and hammer, repeat….they all eventually came out.  Just keep at it and it will work.

Continuing on….depending on the bung and how it was glued in, you may be able to skip the next steps and just use your chisel.  Some bungs that are not solidly glued in will come out with a little bit of effort on the chisel.  With the chisel inside your pilot hole, you might be able to wedge it underneath the bung and carefully pry it out.  Sometimes you might even be able to break up the bung with the chisel in the pilot hole and easily remove it.  However, most of my bungs were glued in and wouldn’t come apart easily so if that is your case, continue on with the next steps.
With the screw head exposed, there should be enough room to insert your driver into the screw slot and slowly, CAREFULLY, back the screw out.  I always test to see if the screw will back out easily turning by hand using either the chisel or another small screwdriver.  Sometimes it will back out, but most of the time it requires the impact driver and all the power behind it. As the screw backs out it will push the bung out with it and if all goes right, create a clean hole.

 With clean holes, you will be able to refinish the wood or whatever need to do and insert new bungs when re-installed.

However, among the thousands of bungs I removed on Windsong, only about 5% would come out easily with this method.  Most of the bungs were glued in solidly and would generally crack and crumble when the screw is trying to pull them out.  Not only that, but if the screw really bites on the bung and pulls it out while solidly glued, it might crack and damage the surrounding wood as it is forced out.  I ruined many a good bung hole (HA!) using this method.
When the bung cracks under this method, a mess is left behind that needs to be cleared out by the chisel.  I find that using the forstner bit to make a large pilot hole for the chisel before anything else is far more efficient and safe for the wood.  As mentioned previously, sometimes all you need is the forstner bit hole and chisel to remove the bung.  If you need more help, at least you are left with a flush pilot hole instead of a chopped up mess.  Through much trial and error, I came to appreciate the method I outlined above.
However, no method is 100% fool-proof.  You will strip many screw heads, and if you do, follow the advice I gave above on slotted screws.  Sometimes no method seems to work well, and you end up just chiseling away at the wood and damage the surrounding area, or you just accidentally ruin the flush hole.  Fear not, as long as it isn’t serious and large damage, you can just drill out a larger bung hole later on.  For example, on all of the bungs I screwed up I plan on drilling out 1/2″ holes to re-plug.

Heap Big Job Mate! A new cockpit floor with Beckson Hatches

 
The original sole looking aft at left

.

A big job replacing the cockpit sole with Beckson hatches  for access. Mark Corke explains how...

When I built Mallard my gaff rigged cutter I was never quite sure how to finish off the sole of the cockpit. On the one hand the cockpit needed to be water tight but on the other hand I also needed to have access for essential maintenance to the stuffing box and primary fuel filter. The other problem was that the cockpit sole was very close to the water line so rigging up the drains so that they would be self draining also became something of a headache, on a larger boat with more free-board the sole is higher and the water can run through drains and exit the boat above the waterline. This was not possible on Mallard so in the end I connected both drains to an inch and quarter sea cock in the bottom of the boat. This worked well enough but it meant that the sea cock had to remain open even when I was not on the boat and this made me nervous not least because if the clamps let go or the sea cock failed water would flood in and the boat would sink. I thought about the problem for several years not quite sure what to do. Finally I came up with the solution that you see in the following pictures, the old sole was completely removed and a now marine plywood sole installed. Two Beckson watertight hatches were fitted to allow continued access to the area under the cockpit and the drains were routed to a self contained sump pump which keeps the cockpit dry and means that the large sea cock can be dispensed with allow me to sleep easier at night.

 Of course every boat will be different and if the hatches are installed in a cored fiberglass deck then I would recommend scooping out some of the core after the cut out has been made and filling with a thicken epoxy to prevent water from migrating into the core material.

 One final point and that is to carefully think through the install before you go cutting into anything; will the hatch interfere with anything above and below decks, will it weaken the boat and do I have the necessary skill set and tools to enable me to complete the installation in professional manner.

Here's how I went about it in words and pictures.

 

Step one was to remove the old drains, which were big, mostly because they incorporate a non return ball valve to prevent water from sloshing back up into the cockpit.

 

The next step was probably the most time consuming of the whole project for me, it was certainly the messiest. Using a Fein multimaster I cut out the existing sole but left the bearers in place as I would need these later to support the new sole.

 

Once the old sole was out of the way I was able to accurately measure for the new half inch marine plywood that would make up the new section. I then cut this out with a circular saw with a fine tooth blade but I could have used a jigsaw. To allow a little room for the epoxy and also to make sure that the ply would not get jammed in I allowed an eighth of an inch clearance all the way around.

 

 

After cutting out the ply to the correct overall dimensions the cut outs for the hatches were marked and then cut out with a jig saw. Like the overall size of the play I made the cut outs and eighth oversize to allow for a little movement.

 

I then bored for the new drains which was much easier to do off the boat. I used a drill press which gave a perfect hole but a hand held drill would have been almost as good had the machine not been available.

 

I used a brass plumbing fitting from my local home store which has a screw on one end and a barb on the other which is perfect for three quarter inch inside diameter hose. With the holes drilled I had a dry run before permanently gluing them in position.

 

Mixing up some epoxy thickened with colloidal silica and wood flour to a mayonnaise consistency I used mahogany dust which gives it the brown color.

 

I then spread a goodly amount onto the screw threads and the inside of the hole and screw the fitting down into position, notice the squeeze out which is ideal. I then left the epoxy to set overnight.

 

With the epoxy set I used an 80 grit disc on the random orbit sander to sand the brass fitting which was slightly proud of the plywood flush. It is important to keep the sander moving for if the brass gets too hot it will soften the epoxy and could weaken the bond.

 

There are two things to do before the the sole can finally be permanently glued in place; drill and countersink holes for the fixing screws and coat both sides of the sole with unthickend epoxy to seal out any moisture.

 

The sole is then ready to be installed and I glued it in place with epoxy thickened to the consistency of peanut butter with colloidal silica which makes a very strong bond once set.

 

With the epoxy still wet the sole is screwed in place. In truth the epoxy is plenty up to the task without using any screws but the screws hold the sole in place whilst the epoxy sets up and don't do any harm if left in position. I used brass screws but if they were in an area that were to be subjected to a lot of sea water than I would have used bronze.

 

With the sole screwed in place I used some more thickened epoxy to fill over the top of the screws and form a fillet (pronounced fill-it) in the corner to provide a smooth transition from the cockpit side to the sole. A maxed out credit card with the corner trimmed to a large radius makes the perfect tool.

 

When the epoxy has set after a day or two the whole sole and fillets were lightly sanded and the paint applied to match the existing surrounding paint. I have found that the best paint over epoxy seems to be the Interlux Epoxy Prime Kote which is then followed up with the colored top coats.

 

With the paint dry the next and final stage of the job can start. The hatches are dropped back into position and the holes for the mounting bolts made. I used an automatic center drill to create a pilot hole. When all these have been completed I removed the hatch once more and set it aside.

 

On the underside of the hatch in hard to read writing is stamped 'use a 12 mm diameter drill'. This is to allow the hatch to move during temperature extremes. It may be tempting to simply fix the hatch in place with some suitable wood screws but it you do there is a good chance that the plastic around the mounting holes will crack as the hatch expands and contracts at different rates to the ply substrate.

 

Using the pilot holes as a guide I drilled the holes to the specified 12 mm with an auger drill.

 

The hatch is ready to be installed but before squirting on the silicone I taped some blue painters tape all around the opening where I knew it would project under the hatch flange. With the hatch dropped back in place a razor blade is run around the perimeter tracing the outline and cutting through the tape.

The hatch is once more removed and the tape on the inside of the cut is peeled off.

 

Some silicone sealant is applied around the entire perimeter.

 

The hatch is then bolted into position with 10-24 stainless steel flat head machine screws, backed up on the underside with fender washers and nuts which are also stainless.

 

Then the perimeter tape is peeled up pulling any squeeze out with it leaving a nice clean and tidy job with no need of any rags or chemical solvents.

 

The finished job all ready for another season. A teak floor grating will be installed in due course.