The books give great how-to info. But there's so much more we needed to know. Friends told us some; other bits we learned for ourselves.

16 Things You Need to Know About Filling In a Thru-Hull

In connection to our conversion from a “wet” marine head to a composting one, we’ve been able to eliminate two thru-hulls. For safety reasons, it seemed smart to us to totally fill in the thru-hulls and not just close the seacock and hope it held. And, of course, we decided to do it ourselves . . .

This isn’t going to be a “how-to” on the project. You can get great information both in Don Casey’s Complete Illustrated Sailboat Maintenance Manual and on the West Systems website. Nope, this is the stuff that either someone did tell us and was extremely helpful or that we wish someone had told us. (Links are to Amazon to show the products discussed in case they’re new to you – several were new to us – but you can get most of the items at hardware and home improvement stores.)

  • Protective gear. Get and wear protective gear from the start: sanding/particle masks, a bunny suit (Tyvek is best but one that’s breathable may be more tolerable in summer heat), goggles, and gloves (latex are fine if you’re not allergic to it). The stuff you’re about to work with is both itchy and hazardous. Dave added a paint spray hood when he was doing the worst of the fiberglass grinding to keep the dust from getting around his neck. Get plenty of masks and gloves – we had 10 masks (about right) and 15 pairs of gloves (could have used more)Grinding out
  • Second person. You can do this single-handed and many portions (grinding . . .) are really one-person jobs. But when you start working with epoxy and fiberglass, a second set of hands is really helpful. The second person will be working with fiberglass and colloidal silica, so get protective gear for them too (if nothing else, make sure anyone who is near the colloidal silica wears a particle mask – it’s bad in your lungs!).
  • Wood plug. If your thru-hull is like ours and has a thick wood backing plate, a good way to fill that hole (not the hole in the hull itself but the inch or so extra through the wood) is to use a hole saw of the appropriate diameter and cut a wood plug that you can epoxy into place. Don’t let it intrude into the fiberglass hole of the hull and use thickened epoxy to secure it. If needed, put a screw into the drill hole left by the hole saw to hold the plug, then after the epoxy has cured, remove the screw and fill that hole with epoxy. Then apply a few layers of fiberglass cloth and epoxy on the inside over the whole backing plate.
  • Showers. You’re going to be working with fiberglass every step of the way, from grinding it to cutting and applying layers of it to sanding it again. The stuff is nasty, itchy and painful on your skin. As soon as you finish for the day, take a cool (not hot!) shower and change into fresh clothes. Don’t wait! (Related and possibly TMI: be careful going to the bathroom that you don’t get fiberglass shards on your genitals. It hurts. Badly. For days.) Do not even think of doing this in a yard without a reasonable shower.
  • Time. This project – filling two thru-hulls – took about 10 days but part of that was that we did it in the middle of summer when it was brutally hot and we just couldn’t work out in the sun more than about 4 hours a day. But some of that time was that we couldn’t apply too many layers of fiberglass and epoxy in a day so that it would cure properly (we did 13 layers a day). Grinding and sanding is hard work, too. And cleaning up each day – both yourself and the work area – takes time. It’s not going to go as fast as you might expect. And you want to do it right – this is a hole below the waterline, after all.
  • How many layers in a day? The books say not to do too many layers at a time due to the heat that builds up in the curing epoxy. There’s also the weight of the wet fiberglass/epoxy and the fact that you’re probably working on a vertical to overhead surface — you don’t want gravity to start making the fiberglass slip (note: Don Casey mentions holding the fiberglass cloth with masking tape when working on a vertical surface; this created a huge mess for us and we found we were better off with just epoxy to hold the cloth). Theoretically, you can do some layers, let it start to cure, do some more, let it start going off, etc. and do the whole layup in a single day without having to do any sanding or between-coats prep. We were exhausted long before then and so had to prep the cured layers from the previous day each morning by sanding and washing with acetone. Typically we did 6 or 7 layers in one hole, then the same in the other hole, took a break, then did 6 or 7 more layers in each hole before calling it a day.

On to tools that we found helpful, tips and amounts of things we used. This list is not complete – you’ll see other things mentioned in the “how to” articles. This is just the extra stuff we wished we’d known . . .

  • Flap disk. The instructions say to grind the fiberglass out to a diameter 12 times the depth you are going to fill. A friend at the boatyard here told us about a great tool to do this with – a “flap disk” (sometimes called a flap wheel) on a buffer/sander. Dave first used a 4” diameter 36 grit one to get things started, then switched to an 8” diameter to finish it. NOTE: Neither Dave nor I had ever seen or heard of these (and my dad owned a hardware store!) so here’s a photo — it was a HUGE help. Ace Hardware had them in stock and so do places like Home Depot and Lowe’s. DIY Filling in a Thru-Hull on a Boat: The books give great how-to info. But there's so much more we needed to know. Friends told us some; other bits we learned for ourselves.Grinding
  • Sander. Because we had to do the fiberglassing over three days, plus a fourth for adding a fill coat at the end, Dave had to sand the previous day’s top layer before starting the next one. Lots of 80 grit sandpaper was used (don’t use the flap disk here, it’s too aggressive) – maybe 30 discs?
  • Acetone. Both to prep the surface each day and to clean us up. Two quarts. And lots of rags and blue paper towels. Plan to throw the rags out; don’t try to wash the epoxy out of them.
  • Brushes. We first tried using throwaway foam brushes. Don’t bother – they’re horrible with epoxy. Use cheap bristle brushes (we used 1” and 1-1/2”) and buy enough that you can use at least two new ones every day. We also had a squeegee and used it “some” but the brushes were the work horse of the project.
  • Epoxy pots. If you mix a fresh batch of epoxy in a pot with some left in it from the previous batch that’s going off, the new batch will also go off almost immediately. (Ask us how we know.) Using a fresh pot stops this. But you can re-use the pots the next day – just pop the cured epoxy out. The pint Mix ‘n Measure pots worked well for us and held up – the brand isn’t important but some are not chemically resistant. Don’t use red solo cups. We used three.
  • Epoxy pumps. Epoxy pumps are worth the money to get the mix right every time. Just leave them in the cans until the project is done; don’t bother to clean them every day as you’ll waste a lot of epoxy, acetone and time. We found that “two pumps” each of epoxy and hardener was about the right amount to work with (your preference may be different but it’s a starting point). NOTE: Different brands of epoxy use different ratios of resin to hardener. Be sure to get the same brand pumps as epoxy.
  • Epoxy amount. Every boat will be different with the size of the holes and the hull thickness, but on a relatively thin-skinned catamaran we used over a quart of resin to fill two thru-hulls (and the corresponding amount of hardener).
  • Fiberglass. Wow! Did we ever underestimate how many layers we’d need – we’d thought at most maybe a dozen, and ended up using 39 per thru-hull! We could only get 6 oz. cloth (not the 10 oz. that Casey recommends) but it was amazing how it thinned out the minute it was wetted with epoxy. We used almost 3 square yards of fiberglass fabric on the two thru-hulls. Others filling thru-hulls in our yard have said the same thing — they expected to use 6 to 12 layers from what they read, and ended up with anywhere from 30 to 60+.
  • Fiberglass circles. Yes, I cut 78 circles from fiberglass cloth.
    • Don’t cut the fiberglass in your lap unless you have several layers of protective fabric between the fiberglass and your legs – a large piece of cardboard is good.
    • The circles will go from the size of the thru-hull to the full circle you ground out. Don’t worry too much about being exact – the fiberglass cloth is very loose weave and pulls and distorts as you put it on and soak it with epoxy.
    • Fold two or three layers of fabric together in quarters and then cut a quarter circle – much faster and easier than trying to cut a full circle.
    • Don’t be surprised if you end up putting smaller on top of larger, etc., as you see how the layup is going. A couple of times we discovered we needed to go back smaller to keep things level.
  • Straight edge. A straight edge that’s long enough to span the entire gound-out area will help as you lay the fiberglass, both to keep it level and to let you know how full the hole is (or isn’t). An old 18″ metal ruler worked well for us. It also helps when you’re doing the final fairing with epoxy/silica and then sanding.

Finally, check with the tech support people for whatever bottom paint you use to determine whether you need to use any sort of primer before applying bottom paint to the “new” hull area.

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  • John Huft
    Posted at 20 August 2015 Reply

    I like the thick nitrile gloves from Harbor Freight. Also dust a little baby powder in the gloves and on your hands before you put your gloves on. The baby powder absorbs the sweat from your hands and the gloves are easy on and off. You can also use baby powder as a thixogen in a pinch.

  • Joie de Vivre
    Posted at 20 August 2015 Reply

    I use Red Solo Cups all the time for epoxy and have not had a problem. The wet epoxy is not in the cup long enough for it to melt the cup. These cups cannot handle the heat of kicking off though.

  • Bert Sailor
    Posted at 20 August 2015 Reply

    Y en a pas de p’tites jobs!

  • Frances Liz Fernandez
    Posted at 20 August 2015 Reply

    Very informative. Not sure we are skilled enough to attempt to take on this project but good to have these tips.

    • The Boat Galley
      Posted at 20 August 2015 Reply

      We’d done a little fiberglass work and some epoxy but we weren’t really experienced. Take it slow and easy and you can do it!

  • Andrew Hogg
    Posted at 20 August 2015 Reply

    I know that getting a boat yard to fix this for you would be very expensive. It looks to me like you now know WHY it’s very expensive. Fibreglass is, as you made sure to highlight, nasty stuff. For all of it’s great properties, it’s dangerous and uncomfortable if not handled correctly. I think you were brave to tackle this job yourself. Well done! And thanks for sharing the tips. After this article, I hope I never need them!

    • Carolyn Shearlock
      Posted at 20 August 2015 Reply

      Yes, and we’re in a do-it-yourself boatyard so no one to hire, really (there is a shop next door but the guy is backed up with work for quite a while).

  • Stephanie
    Posted at 20 August 2015 Reply

    Carolyn, You and your husband do an amazing job at whatever you set your mind to! Love these explanations- SO helpful for anyone attempting this!

    • Carolyn Shearlock
      Posted at 20 August 2015 Reply

      Thanks! I try to pass on all the info I wish I’d found before we started. 🙂

  • Melissa White
    Posted at 20 August 2015 Reply

    Great information! Galapagos has way too many thruhulls. I lost count. One of our goals in taking care of her is to reduce the number of these holes. I’m bookmarking this one!

  • Brian Hollinger
    Posted at 20 August 2015 Reply

    Thank you for this. I don’t like where any of my thru-hulls are and will eventually be moving, or filling, *all* of them…

  • Desiree Golen
    Posted at 21 August 2015 Reply

    We learned that coating our arms, legs and neck with baby powder before every project was a great way to keep those inevitable shards from getting on our skin!

  • Kimberly Boneham
    Posted at 21 August 2015 Reply

    This is very timely as we’ll be starting this exact project next weekend. Thank you!

  • Helen Murdoch
    Posted at 25 August 2015 Reply

    Next time try white vinegar instead of acetone, especially for cleaning your selves. The effectiveness of this versatile product is amazing. We are amatuer boat builders and intitially only used acetone. Now we only use acetone when a tool is required to be cleaned and immediately used as the acetone drys quickly. Friends that have helped are amazed at the effectiveness of vinegar.

  • Daniel
    Posted at 27 August 2015 Reply

    Great info.
    I have two seacocks to remove and patch up that used to go to the old head. (removed for an Airhead)

    • Carolyn Shearlock
      Posted at 27 August 2015 Reply

      You’ll do okay . . . it’s not horrid, just a lot of stuff we wish we’d known before we started.

  • mike hogan
    Posted at 18 August 2016 Reply

    Any reason you choose to use epoxy instead of fiberglass resin? resin cures much faster(typically) than epoxy and i have always been told that it is better to go back to use the same type material when possible.

    • Carolyn Shearlock
      Posted at 18 August 2016 Reply

      Epoxy has the best adhesion to the existing surface. Our boat was originally built with vinylester. In general, “fiberglass resin” (Bondo) is not used for boat repairs. Click here for a good article by Don Casey, author of “This Old Boat” on the differences.

  • Becky
    Posted at 01 September 2016 Reply

    Imagine our horror at discovering during a haulout to prep and bottom paint our Ericson 32…. I was wet sanding the bottom, when I discovered a circular ring in the thin bottom paint, that was oozing water. This was below the head, below the waterline. My husband and I investigated, and concluded what happened in the Calif. construction yard: some worker drilled for a thru-hull, then found out it was in the wrong place. He merely replaced the plug with common bathtub calk, and hid it with the first coat of bottom paint. The only thing holding that plug in after the calk failed was the bottom paint! Imagine if it was pushed inside at sea!!!! We almost fainted, and did a proper fix. Horrors.

  • Ucluelet Charters
    Posted at 02 November 2016 Reply

    Great article looking to fill in a couple through hulls on one of our boats. Live bait wells we don’t use out this way better used for storage.

  • Stuart G Cole
    Posted at 17 November 2016 Reply

    There is no reason to use epoxy in this case, or in most boat repairs, yet amateurs seem to gravitate to it because of marketing, mostly. Its equivalent to using titanium to repair your car body. Polyester resin is far easier to use, and its what the boat is made of (okay vinylester, so get vinylester resin if you want – the stress on this area is negligible). One can control the thickness (run, sag), and the cure time. And if you use an air inhibited laminating resin it eliminates sanding between coats, a huge time saver. Also get some thick biaxial mat/cloth. Granted I have long experience in fiberglassing boats, but this project would take me about 3 – 4 hours max including final sanding and coating, if I really cared since it will be covered by bottom paint anyway. I do like your tip to use a flap disk, that is great.

  • Kim Gibson
    Posted at 05 July 2017 Reply

    Wondering if there’s a way to save this post!

    • The Boat Galley
      Posted at 05 July 2017 Reply

      Bookmark it, or just know that it’s on The Boat Galley and you can use the search feature on the site to find it again. Also, supposedly if you SHARE it here on Facebook it will be saved on your timeline.

  • Lindy Duncan
    Posted at 05 July 2017 Reply

    You can “save a post” on fb. In the right hand corner above the picture, there is a little arrow. Click on it and a dropdown menu appears. You should have the option to save the post there.

  • Montana Inkfish
    Posted at 06 July 2017 Reply

    Great overview. Thanks!

  • Jeff Ferguson
    Posted at 06 July 2017 Reply

    I decommissioned one simply by securing plugging the bronze thru hull. Does the job and the thru hull might come in handy for something else down the road.

  • Matthew Bastian
    Posted at 23 August 2017 Reply

    Epoxy pumps are not as accurate as you might think! They do a good enough job that your epoxy will cure and you won’t notice the difference but if you really want a 100% ratio dispense by wight or a graduated measuring device.

    If you want to be shocked, try pumping into container sitting on weigh scale and note the variations from pump to pump. The first pump of a session will always be the worst. Pour it all back when done the experiment.

    BTW. Really enjoy your site and find it very helpful. I consult it regularly.

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