How to remove bottom paint?

[Ryan61065]

Hello fellow sailor's,
Been getting back into my restoration project of a 1983 Hunter 20. My problem is that I'd like to possibly race this boat and from what I've seen on the YouTubes most trailer sailor racers have nice buffed gel coat below the water line. Unfortunately for me my boat has a nice old layer of ablative anti fouling paint. Now I have no doubt that a little paint stripper will remove the majority of the old paint, however I doubt it's gonna remove all of it. I could always try and sand the remaining paint off but then I'm scared I'm gonna burn through the gel coat. I've looked into non anti fouling bottom paint but it doesn't seem to exist unfortunately. Has anyone successfully removed all the old bottom paint off their boat in an effort to polish and buff the existing gel coat underneath? I'd really like a slick surface under the water line and I just don't see the point to anti fouling paint on a trailer sailor, especially in fresh water. The longest I can imagine it being in the water is a weekend
Thanks!
Ryan

 

[twodzusfittings]

Have you thought about Soda blasting? Once it has been brought down to the gel coat, you will need to check and repair any blisters, gouges and light scratches. After that a great primer and paint job with a high quality paint, more than likely a two part product to achieve what I believe your vision is. Doesn't come cheap. There is a lot of getting the correct conditions. I would weigh the cost vs. desired outcome/benefits to make the decision.

 

[rgranger]

What I am about to share, I've actually done about a dozen times.... Go to Lowes or Hope Depot and get a 5lb bag of garden lime (poweder). Put that in a 5 gal PVC bucket and then add four or five cans of drain-o (or other source of pottasium hydroxide --- sodium hydroxice also works). You need about four soup cans of a hydroxide salt. Other sources of hydroxide salts are canning lime and most drain cleaners. Add just enough water so that it stirs like hot peanut butter. Put a roller on a stick and Roll this onto the bottom of your boat at dusk and let it sit over night. Power wash it off. If you lay an old sheet under the boat, it will act like a filter you can capture most of the paint flecks. You may need to repeat on a few tough (thick) spots but you will get most of it off on the first pass.

DO NOT LET THIS GET IN YOUR EYES!!!! Wear eye protection!!! Wear long sleeve shirt while rolling it on. It will leave you with a chemical "burn" if you get it on your skin. If you do get it on your skin. Wash with a lot of water before a rince with vinegar. Then rub some vasaline or olive oil onto the spot of skin. This mixture will essentially turn the oils in your skin into soap and then you will get the worst chapped skin you have ever had... so the vasaline or oil will replace the lost natural oils in your skin.

I have also wondered if you could just use liquid plumber but I have not tried that yet. If you try it, let us know how it works.

 

[American_Mainsail]

What I am about to share, I've actually done about a dozen times.... Go to Lowes or Hope Depot and get a 5lb bag of garden lime (poweder). Put that in a 5 gal PVC bucket and then add four or five cans of drain-o (or other source of pottasium hydroxide --- sodium hydroxice also works). You need about four soup cans of a hydroxide salt. Other sources of hydroxide salts are canning lime and most drain cleaners. Add just enough water so that it stirs like hot peanut butter. Put a roller on a stick and Roll this onto the bottom of your boat at dusk and let it sit over night. Power wash it off. If you lay an old sheet under the boat, it will act like a filter you can capture most of the paint flecks. You may need to repeat on a few tough (thick) spots but you will get most of it off on the first pass.

DO NOT LET THIS GET IN YOUR EYES!!!! Wear eye protection!!! Wear long sleeve shirt while rolling it on. It will leave you with a chemical "burn" if you get it on your skin. If you do get it on your skin. Wash with a lot of water before a rince with vinegar. Then rub some vasaline or olive oil onto the spot of skin. This mixture will essentially turn the oils in your skin into soap and then you will get the worst chapped skin you have ever had... so the vasaline or oil will replace the lost natural oils in your skin.

I have also wondered if you could just use liquid plumber but I have not tried that yet. If you try it, let us know how it works.

Very interesting. Do you have a source for this formula? What are you ultimately making here?

This is a really excellent question. Walking around boatyards, you see years of built-up ablative paint on many (if not most) of the boats in the yard. Fairing out old bottom paint takes a lot of work but, if you have the tools and stamina to do it, its very much worth it in the performance you get back.

I have used sand paper and an electric DA vacuum sander attached to a small generator. 80 grit sandpaper is the standard, but you can to a lower grit for more abrasive action. I've had success using this sander (and wow! the price has come down by half since I bought it):
https://www.amazon.com/MAXXT-Brushless-Multi-function-Woodworking-Polishing/dp/B0BFWK37Q6/ref=sr_1_1_sspa?crid=10ZO5DNK4IL4N&dib=eyJ2IjoiMSJ9.omA3iy1vXkpat6ZIdvLDTTNbc281XcnVijtjz1lOzKoq_lS9vhqJC94RSwZTSE5u42MSeqbr_KHPI63hgLRZhT7e-XppoQ_c4UuOUgVJ_8iRWP-86i2C_PdzvGFIONzoDErbn567ki1p5JW_m7AC9pmPGBExDl25yPMHN7vEz7m3erINJdDM2BG0m_AZmE0hTEu0oN9X1nvLRCXWVhfsJ_qdV66A2rx3_RomPgQ09meOYDQ58Vvi85arrvBgqDZJXdpEt1MsbYZphb-CduYT1BFjsOP-yDKvy5XpSU_ZaUQ.S1MLCShuym8bDOmMbHl6TdzZfPQaDpf5g9w3E1Jd8iw&dib_tag=se&keywords=dual+action+electric+sander&qid=1772807075&sprefix=dual+action+electri+sander,aps,160&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1

Quality abrasives are key. I found that 3M Cubitron II was one of the best performing papers that I tried, but I wasn't able to find anything less than 80 grit of this type. It might be worth it to get a more aggressive type of paper from a quality manufacturer for areas that are particularly tough. Cheap bottom paint can be "gummy" and can easily clog the sandpaper. Tuning in the right speed on the sander is important to avoid gumming the paper.

This project is a lot of work, but will be manageable in a weekend on a 20' boat.

Edit: rereading your original question and seeing that you're planning on trailering this boat! I see what you're trying to do now. I think a chemical stripper, soda-blasting, or hiring a professional to do the removal would be best, as other posters have suggested. Sanding down to bare gel coat will leave a lot of scratches on the bare gel coat.

There are specialized tools out there that professional yacht care companies can deploy to achieve perfect removal of everything down to the gel coat. This pulls everything off, however (including epoxy barrier coat, if you have one), which is why I tend to default to sanding unless we're talking about starting totally from scratch.

 

[Stu Jackson]

Walking around boatyards, you see years of built-up ablative paint

Did you mean hard paint? Ablative sloughs off, doesn't it?

 

[American_Mainsail]

Did you mean hard paint? Ablative sloughs off, doesn't it?

No - I meant both. In my experience with ablative paints, when they're at the end of their life they leave some resin/matrix material behind that is really stiff and won't come off without some kind of chemical intervention or sanding. The really good, expensive paints do this to much less of a degree than the cheaper ones.

I'm curious if you haven't noticed this on your boat. If you haven't, what kind of paint do you use that isn't building up at all?

 

[Stu Jackson]

I'm curious if you haven't noticed this on your boat. If you haven't, what kind of paint do you use that isn't building up at all?

I've used Petit Trinidad since 1998 in the US until I moved here in 2016, then another hard paint in 2020, only used ablative in late 2025, so I don't know the answer. Just had years of paint buildup that never concerned me at all.

 

[GeorgeC]

I would be EXTREMLY CAUTIOUS in using any product with Draino in it.

I stripped the bottom down to the gel coat on my O'Day 30 years ago. I used a product called Peel Away and it did a great job and the bottom looked like it did when my boat was delivered from the factory. The problem I had was that the product that managed to make it past my rubber gloves and down my arms caused the burning of the skin. It also makes a mess on your clothing and the ground below the boat. I would expect that most marinas today will require that you put a tarp under the hull to catch all of the product and paint that comes off the hull. At the same time i was doing my boat another boat owner had the yard strip the paint off the bottom of his boat with walnut shells. His bottom did not look as good as mine but he did get the paint off the boat. If I was going to strip the paint off again I would have the yard use some form of walnut shell blasting to remove the paint.

 

[jssailem]

There are some jobs that DIY makes sense. There are some jobs that involve chemical exposure better left to others with experience and materials/equipment.

Captains/boat owner get to decide.

 

[rgranger]

Very interesting. Do you have a source for this formula? What are you ultimately making here?

It’s basically (pun intended) the same active ingredients used in Peel Away: calcium hydroxide (slaked lime) at roughly 20–50% and sodium hydroxide (lye) at about 1–20%.
You can add binders like sugar or a water‑soluble glue to keep the mix moist and reactive longer. Over time, I stopped using binders and simply misted the paste a few times to re‑wet and re‑activate it before pressure washing.

As @GeorgeC mentioned, you do not want this on your skin or in your eyes. Wear long sleeves and eye protection, keep a hose nearby, and just use common sense.
People will point out the risks, but in my experience the dust from sanding is much harder to contain than the residue from a chemical peel.

When I used to do this in my yard, I’d lay an old bed sheet under the boat before pressure washing. It worked like a giant coffee filter, catching most of the copper paint chips and residue and keeping them out of the soil.
You’ll be amazed that people still sand bottom paint. This method saves hours of back‑breaking work and costs less than $20. I’d apply the paste with a paint roller on a long pole, let it sit overnight, mist it in the morning, and pressure wash an hour later. Typically, only two or three small spots wouldn’t react. Sometimes a second application worked; other times I’d use a little Citristrip and scrape those stubborn areas.
Before repainting, rinse the hull thoroughly, mist it with diluted vinegar using a garden sprayer, then rinse again. This neutralizes any remaining alkalinity and balances the surface pH before painting. The vinegar step is probably overkill, but it’s cheap and quick.