Through Hull Replacement, Part 9: Resizing the Large Chamfered Hole

Sanding the cured epoxy in the large chamfered hole
When resizing the chamfered hole for flush-mounted through-hulls, you have two options. Either you can coat the new through-hull with wax and screw it into place overnight, hoping that the wax will prevent the thickened epoxy from sticking to the bronze, or you can coat the new through-hull with wax and screw it into place temporarily, hoping that a good impression of the head of the new through-hull will be left behind in the still-uncured epoxy. I describe both of these techniques in greater detail in Part 4 of this multi-part article on through-hull replacement. If you have read Part 4, "Resizing the Holes," you'll know that I favor the latter technique, if only because I don't feel comfortable allowing a bronze through-hull to sit overnight in a bed of epoxy. You'll also know that if you use the latter technique, you'll need to do a good bit of sanding to make the new chamfered hole smooth and consistent, since the impression left in the uncured epoxy is not entirely smooth and consistent.

In Part 4, "Resizing the Holes," I described how I resized two different small chamfered holes to accommodate new, 3/4 inch, Groco brand bronze through-hulls; I also mentioned there why it was unnecessary for me to resize the large, waste-outlet through-hull, since the head of the new 1.25 inch Groco brand through-hull fit well into the original chamfered hole. At the time, it did indeed seem that the resizing of this large hole was unnecessary. After, however, I had gotten to the point where I could dry-fit all of the new hardware, I had come to the realization that the head of the new through-hull did not fit as well as I thought it did. This meant that, before I could do anything else, I had to resize this large hole.

For the resizing of this large chamfered hole, I would use the same technique that I used for the smaller holes. I would, however, develop a new way of ensuring that the new hole was as smooth and consistent as possible. This new approach that I took for the resizing of this large hole in Oystercatcher, my Ericson 25, is the subject of this penultimate posting on through-hull replacement.
The original chamfered hole
You'll know from my previous posting that I experienced some difficulties when drilling the bolt holes for the flange (the piece of hardware into which the through-hull is screwed). The wooden bulkhead, beside which the backing plate and flange are located, prevented me from situating myself and my tools squarely over the flange. Consequently, I had to hope for the best when drilling the holes. I was fortunate that two of the three bolt holes (pictured below, top and right) exited the hull in the proper location. One of the bolt holes (at the bottom of the picture), however, did not. This caused head of the bolt, when inserted into the hole, to encroach on the head of the through-hull. As I said in the previous posting, I had to re-drill the hole from a different angle, in order to get everything to fit just right.

When I finally did get everything to fit just right, I was a bit troubled by the fact that there was now a noticeable gap between the edge of the head of the through-hull and the hull itself. I was also troubled by the fact that the bottom bolt hole, due to my re-drilling of the hole, was larger than it needed to be. For this reason, I decided that I had no choice but to remove the hardware and fill both the chamfered area and a portion of the re-drilled hole with thickened epoxy.
As I had done when I had resized the smaller holes, with this large one I thickened some epoxy with colloidal silica and spread it around with a plastic stir-stick. In the picture below, you can see how I used the epoxy to fill a portion of the bottom hole. Notice how the top two holes are countersunk. Understandably, I decided to postpone the countersinking of the bottom hole until after this epoxy work was complete.
A few days later, after the epoxy had cured, I broke out the Dremel and got to work. As I have said elsewhere, this tool, especially when equipped with the right-angle attachment, is incredibly helpful for all sorts of boat projects.
After I had removed some of the hardened epoxy with the Dremel, I checked the fit of the through-hull.
Obviously, there was still a lot of material to be removed, but the last thing I wanted to do was to remove too much of it. If I did, I would have to start all over.
As I worked my way deeper into the cured epoxy, I created more and more dust. Despite the fact that it was cured and thus much less dangerous than it would be in its uncured state, I wore an organic vapor respirator. I've seen some guys in boatyards sand this stuff and smoke cigarettes at the same time. Not me.
Normally, I wear gloves when sanding cured epoxy, but for this job I needed to feel the contours of the chamfered hole.
Eventually, it seemed that I had reached the end of the road with this sanding job. The hole appeared to be smooth, consistent, and well-chamfered.
Nevertheless, when I screwed the through-hull into place, I could still detect some small gaps here and there between the bronze and the hull.
Therefore, I removed the through-hull and did a little more sanding.
Again, however, when I screwed the through-hull into place, I could detect some gaps. These gaps were in the same place, despite the fact that I had just sanded those areas. This made me think that perhaps there was some other area of the chamfered hole that was causing this area to have the gaps. This also made me worry that if I continued to sand in the same area that I had been sanding (which, incidentally, had been in the upper left area of the hole), then I might remove too much material and thus cause the entire job to be ruined.
It was at this point that a simple, yet effective, idea came to me. What if I covered the flange with ink from a red Sharpie marker, and then screwed the through-hull back into place? Wouldn't that reveal the high spots in that chamfered hole? That's what I was thinking, so I thought it wouldn't hurt to try.
I screwed the through-hull into place, and then unscrewed it. Not surprisingly, the ink revealed that the problem area was not in the upper left (where I had seen the gap, and where I had been sanding), but in the lower right portion of the hole. I therefore began to sand this area.
Again I screwed in the through-hull and then unscrewed it. Again, the red ink indicated that the lower right area of the hole was the problem.
I repeated this process, and found similar results.
The next time, the problem area appeared at the bottom of the hole. This meant that the through-hull would surely now fit more snugly in the hole, since I had eliminated the first high spot.
Now I would start working on this bottom area. I'll spare you the many photos I took (for the purpose of reminding myself how many times I had to do this), but I will say that I repeated this process seven more times.
Despite my repeated sanding of this bottom area, I was still finding it to be problematic. Therefore, I took a short break, re-coated the head of the through-hull with red ink, and got back to work.
The first two times I screwed in the through-hull and unscrewed it, I continued to find marks at the bottom of the hole, such as you see below.
The third time, I began to see marks again at the lower right portion of the hole. This meant that I had solved the problem at the bottom. The better fit of the through-hull, after I had screwed it back into place, only confirmed this.
Seven more times I would do this, and each time I would find the marks in the same, lower-right area.
Again I stopped to take a break and to re-coat the head of the through-hull with ink.
This time, I began to see the red marks more evenly distributed along the lower half of the hole. This was a sure sign that I was getting close to the end.
Three more times I did this until, at last, the marks were scattered here and there around the hole. Finally, it appeared that I had sanded this hole as smoothly and consistently as possible under the circumstances. When I screwed the through-hull into place, there were no gaps whatsoever between its head and the hull. This confirmed what the red marks on the inside of the hole had revealed.
Sometimes I have wondered why this large chamfered hole was more difficult to shape than the smaller ones. I believe it was simply a matter of the size differences of the holes. For me, it was  easier to sand the smaller holes in a consistent pattern. The larger hole, I believe, allowed for more variations in the angle of attack. Without using this red-ink method, I'm not sure that I could have achieved a snug fit.
Despite the fact that I had to screw, unscrew, and sand this large hole many times, I'm still glad that I approached this project in this fashion. Sure, I could have coated the through-hull with wax, screwed it into place, and allowed it to remained seated overnight while the epoxy cured. Some persons have apparently found success with this method, but this is not a method that made me comfortable. I could not risk permanently bonding the through-hulls to their bronze flanges with epoxy.
This ends this posting on how I resized the large chamfered hole for the new, 1.25 inch, waste-outlet through-hull for Oystercatcher, my Ericson 25.

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