Filling in a thru-hull is a straightforward fiberglass and epoxy project, but it takes significantly more time, materials, and layers than most boaters expect going in. When Dave and I did it on our Gemini 105 catamaran, we learned plenty of things the hard way — stuff the books don’t cover and that other boaters in the yard gave us only in bits and pieces.
This isn’t a step-by-step how-to. For that, Don Casey’s Complete Illustrated Sailboat Maintenance Manual and the West System website both give excellent technical guidance. What we’re covering here is everything else: the supplies you’ll go through faster than you think, the tools that made a huge difference, and the things that’ll save you time, skin, and a lot of frustration.
Protective Gear: Get More Than You Think You Need
Fiberglass is itchy, sharp, and hazardous. The materials involved in filling a thru-hull are genuinely unpleasant on your skin, in your lungs, and in your eyes. Get proper gear before you start — and get more of it than you think you’ll need.
You’ll need:
- Sanding/particle masks (Amazon)
- Tyvek bunny suit (Amazon)
- Safety goggles (Amazon)
- Latex or nitrile gloves (Amazon)
Dave also wore a paint spray hood (Amazon) during the worst of the fiberglass grinding to keep the dust from getting around his neck. It worked well.
We went through about 10 masks and 25 pairs of gloves on a two-thru-hull job. Buy more than you think you need.
You Really Do Want a Second Person
You can fill in a thru-hull solo — and some steps, like grinding, are really one-person jobs anyway. But once you’re working with wet epoxy and fiberglass cloth, a second set of hands makes a real difference.
Just make sure that second person is also suited up.
Fill the Wood Backing Plate First
If your thru-hull has a thick wood backing plate (ours did), you’ve got a hole through the wood to deal with before you even get to the fiberglass hull itself. We decided to fill that hole as well as the one in the fiberglass as sort of a “belt-and-suspenders” move to be sure the hull wouldn’t leak.
The easiest way to handle it: use a hole saw of the right diameter to cut a wood plug. Use thickened epoxy to secure it in place — and if you need something to hold it in your fingers while positioning it, run a screw into the drill hole left by the hole saw. Once the epoxy has cured, remove the screw and fill that hole with epoxy too. Finish by laying a few layers of fiberglass cloth and epoxy over the whole backing plate on the inside.
Shower Immediately After Every Work Session — With Cool Water
Fiberglass gets into everything. The moment you finish for the day, take a cool shower (not hot — heat opens your pores) and change into clean clothes. Don’t wait, don’t sit down first, just go.
A note that’s slightly TMI but genuinely useful: be careful not to transfer fiberglass shards when using the bathroom. It hurts. Badly. For days.
Also: don’t even consider doing this project at a boatyard that doesn’t have decent showers.
Budget More Time Than You Think
We filled in two thru-hulls in about 10 days. Part of that was the brutally hot summer conditions — we could only work about 4 hours a day in direct sun.
But even in better weather, this isn’t a weekend job. Grinding and sanding are hard physical work. Cleanup — both yourself and the work area — takes real time every day. And you can only apply so many fiberglass layers per day before the heat buildup from curing becomes a problem.
This is a hole below the waterline. Do it right, not fast.
How Many Fiberglass Layers Per Day?
The books warn against applying too many layers at once because of the heat that builds up as epoxy cures. There’s also the weight of wet fiberglass on a vertical surface — gravity will eventually start winning.
In theory, you can do multiple rounds of layup in a single day: apply several layers, let them start to go off, apply more. We were too exhausted to manage that. Instead, we did 6 or 7 layers in one thru-hull, then the same in the other, took a break, then repeated — 13 layers total per day per hole. NOTE: The top layer from the previous day must be sanded and wiped down with acetone before starting again (this is only required before starting each day, so that the epoxy will adhere to the now-cured epoxy from the day before).
Casey’s book mentions holding fiberglass cloth against vertical surfaces with masking tape. We tried that, and it created a mess. Just the epoxy worked better for us.
The Flap Disk: A Tool You Probably Haven’t Heard Of
The instructions say to grind the fiberglass out to a diameter 12 times the depth of the hole you’re filling. The tool that makes this possible — and that neither Dave nor I had ever encountered before — is a flap disk (sometimes called a flap wheel) mounted on a buffer/sander.
Dave used a 4″ diameter, 36-grit flap disk to rough things in, then switched to an 8″ diameter to finish. It was a huge help. Most hardware stores, plus Home Depot and Lowe’s, typically stock them.
Sandpaper: Plan on More Than You Expect
Because we spread the fiberglassing over three days (plus a fourth day for the fill coat), Dave had to sand the previous day’s top layer each morning before starting. We went through about 30 discs of 80-grit sandpaper.
Don’t use the flap disk for this step — it’s too aggressive. Stick with regular sandpaper.
Acetone and Rags
You’ll use acetone both to prep the surface before each day’s work and to clean up afterward. We used about two quarts. Stock up on rags and blue paper shop towels, and plan to throw the rags out rather than trying to wash epoxy out of them. Always wear gloves when using acetone.
Use Bristle Brushes, Not Foam
We started with throwaway foam brushes. Don’t bother. They’re useless with epoxy. Cheap bristle brushes work far better. We used 1″ and 1-1/2″ widths and went through at least two fresh brushes every day. We also used a squeegee occasionally, but the brushes did the real work.
Fresh Epoxy Pots Every Batch
If you mix a new batch of epoxy in a pot that still has old epoxy starting to cure in it, the new batch will also kick off almost immediately. (We learned this the hard way.) Always start each new mix in a clean pot.
That said, you can reuse the same pots the next day. Just pop the cured epoxy out. The pint Mix ‘n Measure pots held up well for us. Don’t use red Solo cups — we found that out the hard way too. Whatever pots you use, make sure they’re chemically resistant.
Epoxy Pumps Are Worth It
West System epoxy pumps (Amazon) are worth every penny for getting a consistent mix ratio every time. Leave them in the cans for the duration of the project — cleaning them every day wastes epoxy, acetone, and time. We found that two pumps each of resin and hardener was a good working amount, but your preference may vary.
One critical note: different brands of epoxy use different resin-to-hardener ratios. Make sure the pumps match your epoxy brand.
Plan for a Lot More Epoxy Than the Books Suggest
Every boat is different, but on our relatively thin-skinned catamaran, we used over a quart of resin (plus corresponding hardener) just for two thru-hulls. Budget generously.
You’ll Use Far More Fiberglass Layers Than You Think
This was our biggest surprise. We expected maybe 10 to 12 layers per thru-hull based on what we’d read. We ended up using 39 layers per thru-hull.
We could only source 6 oz. cloth rather than the 10 oz. Casey recommends, and 6 oz. thins dramatically once wetted with epoxy. We used almost 3 square yards of fiberglass fabric on two thru-hulls. Other boaters in our yard reported the same experience — they’d budgeted 6 to 12 layers and ended up using 30 to 60+.
Buy more fiberglass cloth than you think you need. Seriously.
Cutting the Fiberglass Circles
Yes, you’ll be cutting a lot of circles — we cut 78 of them. They graduate in size from the diameter of the thru-hull up to the full diameter of the ground-out area.
A few things that help:
- Don’t cut fiberglass in your lap, no matter what you are wearing as protection. Use a large piece of cardboard as a cutting surface.
- Wear gloves any time you handle fiberglass cloth.
- Fold two or three layers of cloth together in quarters, then cut a quarter circle — much faster than trying to cut full circles one at a time.
- Don’t stress about being exact. Fiberglass cloth is a loose weave and distorts when you apply epoxy anyway.
- Don’t be surprised if you end up going back to a smaller circle mid-layup to keep things level. It happens.
Use a Straight Edge to Monitor Your Progress
A straight edge long enough to span the full ground-out area lets you check whether the surface is staying level as you build up layers — and tells you how far you are from flush. It also helps during final fairing with epoxy/silica. We used an old 18″ metal ruler, but even a piece of dowel would have worked.
One More Step After the Epoxy Cures
Before applying bottom paint to the repaired area, check with the tech support line for your specific bottom paint to find out whether a primer is required over the new fiberglass. Don’t assume — some paints need it, some don’t.
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Carolyn Shearlock has lived aboard full-time for 17 years, splitting her time between a Tayana 37 monohull and a Gemini 105 catamaran. She’s cruised over 14,000 miles, from Pacific Mexico and Central America to Florida and the Bahamas, gaining firsthand experience with the joys and challenges of life on the water.
Through The Boat Galley, Carolyn has helped thousands of people explore, prepare for, and enjoy life afloat. She shares her expertise as an instructor at Cruisers University, in leading boating publications, and through her bestselling book, The Boat Galley Cookbook. She is passionate about helping others embark on their liveaboard journey—making life on the water simpler, safer, and more enjoyable.
John Huft says
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 says
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 says
Y en a pas de p’tites jobs!
The Boat Galley says
C’est vrai!
Frances Liz Fernandez says
Very informative. Not sure we are skilled enough to attempt to take on this project but good to have these tips.
The Boat Galley says
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 says
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 says
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 says
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 says
Thanks! I try to pass on all the info I wish I’d found before we started. 🙂
Melissa White says
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 says
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…
The Boat Galley says
Ugggh . . . big job!
Desiree Golen says
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 says
This is very timely as we’ll be starting this exact project next weekend. Thank you!
Helen Murdoch says
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.
Carolyn Shearlock says
Thanks — we’ll try it.
Daniel says
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 says
You’ll do okay . . . it’s not horrid, just a lot of stuff we wish we’d known before we started.
mike hogan says
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 says
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 says
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 says
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 says
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.
Carolyn Shearlock says
We went with epoxy on the advice of Don Casey as the resin is also gluing the patch to the nearby surface. Here’s Casey’s analysis of which to use when:
http://www.boatus.com/boattech/casey/polyster-epoxy.asp
Jeanie B. says
Great post, Carolyn! I worked in a boat building and repair yard and for this repair we also would use polyester resin. We use epoxy/fiberglass layers for gashes in hulls, etc. But good on you, those areas are now stronger than original. Ditto on the flap disk.
Kim Gibson says
Wondering if there’s a way to save this post!
The Boat Galley says
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 says
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.
The Boat Galley says
Thanks!
Montana Inkfish says
Great overview. Thanks!
Jeff Ferguson says
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 says
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.
Ritchard Findlay says
I have done a number of thru hull jobs like this. One trick I use is to go grab a piece of the heavy plastic sheet that we use to wrap boats in winter in the north – you southerners can find some other heavy piece of plastic sheet. When you are ready to place your fibreglass patches, wet out the hull a couple of inches larger than the hole. Place your patches per normal, then cover them with a chunk of the heavy plastic that same 2 inches larger than the repair. The plastic sticks to the epoxy tenaciously, holding the patch in place, but it will peel off fine in the morning. The excess epoxy that held the plastic sands away easily.
The Boat Galley says
Great idea!
Dave Chittleborough says
Why not do it properly and use peel ply. Then you don t need to sand before painting 😜
Ritchard Findlay says
Dave Chittleborough If I had peel ply, I’d use it. I have shrink wrap.
Pierre Mitham says
Dave Chittleborough Peel ply doesn’t prevent the need for smoothing completely but it does greatly reduce it. the West guide on this is great!!
https://www.youtube.com/watch?v=6N7YMr6E564
Ritchard Findlay says
But come to think of it, shrink wrap would work better for this purpose than peel ply. Much much heavier gauge.
Pierre Mitham says
Ritchard Findlay if you can’t find peel ply, go to fabricland and by a couple of yards of polyester dress lining. Works exactly the same and only costs about $5 a yard!
Pierre Mitham says
Seems like a long time (10 days) to fill in a thruhull. With slow set hardener I was able to do 7 in 2 days. smend the morning with the grinder and flap wheel to create the tapers in the patch, then the afternoon to cut the patches (I used biaxial 1708 for 15 layers on my hull) then the next day, apply the internal partches, then grind and smooth the outside to a finish for barrier coat.
The Boat Galley says
If we ever do it again, we’ll use heavier fiberglass fabric if we can get it. Nowhere nearby had it, so we used thinner fabric. We would have been a lot better off to order thicker and wait for it.
Pierre Mitham says
1708 is much easier to work with, stronger and neater. 15 layers is almost 3/4 inch thick!
The Boat Galley says
I keep learning . . . thanks for all the better ways to do it! More stuff that’s not in the books and websites I was referring to!
James Delane says
Acetone is not very healthy. Also, when you use acetone to clean any epoxy that gets on your skin, it leaves a sticky residue. If you accidentally get epoxy on your skin, use vinegar to get it off. It is easier on your body and leaves no residue. Clean the epoxy off with vinegar and clean the vinegar off with a nice smelling soap and water.
Tami says
THE bible:
https://www.westsystem.com/wp-content/uploads/Fiberglass-Manual-2015.pdf
Paul Mac Menamin says
One thing I found very helpful was to get a “Easy Cutter” electric scissors. Makes cutting 1708 a breeze. Available from Jamestown Distributors
Scott says
Be VERY careful with vinegar inside a boat. Vinegar can and will soften or dissolve cured resin as well as uncured. Clean up any spills immediatly, rinse and clean up the water.