The Scraper-only Approach to Bottom Paint Removal

Spring is just around the corner. For many sailors, especially those with boats on the hard, it’s good time to consider the advantages of stripping off years of old paint.

Bottom paint can begin to lose adhesion after as few as eight coats of ablative paint and ten coats of hard paint. When this happens, bottom paint flakes away, and the hull is no longer fair or fully protected. Fouling begins and your boat speed suffers as drag increases.

Applying more paint over a surface pocked with craters increases drag and continues to degrade the boat’s performance under sail or power. Most sailors tend to use one of three basic approaches to removing all their bottom paint: several days of sanding; supplementing their sanding efforts with costly, slow, fussy, chemical strippers (many with narrow temperature windows); or the most expensive option, hiring a professional to media-blast the bottom clean, so it’s ready for barrier coat and new antifouling paint.

As part of the rebuild of my Cape Dory 36 Far Reach (see “Rebuilding a CD 36,” PS Nov 2022 to PS Mar 2023), I had sanded off all the old bottom paint (there was no barrier coat then) applied new barrier coat and antifouling paint. Eleven years later, when I repowered (see “Marine Diesel Repower from Scratch,” PS April–May 2023), I decided it was a good time to strip the antifouling paint down to the barrier coat and reapply fresh barrier coat and antifouling paint before we relaunched the boat.

Being the committed DIY type, and on a tight budget, I always look for the least expensive way forward. I’m always willing to trade my labor to save money. Even though I am no longer a youngster, I consider myself fit, tough, and committed. I started this project thinking I would sand off the old paint. After all, I’d done it 11 years earlier.

I was quickly served a dish of humble pie. The incredibly tough layers of antifouling paint had bonded to the barrier coat. It was just too difficult to sand it off. Luckily, I stumbled onto another approach I was not familiar with—using a scraper as the primary means to remove the antifouling. It worked surprisingly well and, though time consuming, was far easier than sanding.

THE TASK

The reason the paint was so much harder to remove this time was because I’d “hot-coated” the first coat of my Interlux Micron CS ablative antifouling to the still tacky barrier coat, as described in the application instructions for Interlux’s Interprotect 2000E, the time-tested epoxy-based barrier coat system I had used. This step essentially bonded the antifouling paint into the barrier coat to ensure the best possible adhesion between the two coatings.

Although we only had about six coats of antifouling paint on the hull, it was strongly attached to the barrier coat. Thus, the test sanding had demonstrated the “hot-coat” approach had worked as intended, creating a strong bond between the paint to the barrier coat.

As I pondered other options, a friend stepped in and suggested using a pull-scraper with carbide blade. I happened to have a Warner 100x pull-scraper with a 23/8-inch-wide double-edged carbide blade in my shop.

My initial thinking was scraping with a blade that small would take forever. Boy was I wrong. I took it to the boat, spread a plastic tarp under the test area and in less than 20 minutes of scraping I had cleaned off a section of antifouling two feet wide from the waterline to the bottom of the full keel.

Best of all, because my hull has long vertical sections, I was able to mostly use my weight and not my biceps and arms to pull down on the scraper, so there was very little fatigue. Unlike a sander, the scraper weighed almost nothing. To my surprise, most of the barrier coat remained on the hull, which I saw as a good thing.

One advantage of scraping is that scraping requires fewer measures to prevent soil contamination with cuprous oxide, the toxic antifouling agent in most bottom paints. If I had sanded the antifouling paint, I would have needed to spread a ground cloth and tent the hull; or spread a ground cloth and use a quality vacuum system on the sander. Boatyards can face hefty fines if the U.S. Environmental Protection Agency discovers elevated levels of cuprous oxide in their soil.

For scraping a hull, all I needed to do was spread a ground cloth and bag the debris.

SAFETY FIRST

In mid-March, while the temps were in the 50s, I purchased about a half dozen double-edged carbide scraper blades from Lowe’s hardware. I spread a folded 12-by-18-foot plastic tarp under the boat where I was working.

Wearing a Tyvek paper suit, half-face respirator, and gloves, I commenced scraping from the waterline to the keel in an organized sequential manner, tackling a 2-foot-wide section at a time.

In 10 hours, spread over three days, I had scraped the hull free of 95-percent of the antifouling.

The only thing scraping failed to remove were scattered clusters of tiny spots of paint tightly bonded to the barrier coat. All of this was removed later by sanding with a random orbit sander (see “Random Orbital Sanders for Bottom Paint Prep,” PS February 2024.)

Interestingly, although scraping removed nearly all the antifouling paint, it did not remove much barrier coat. (If you want to also strip the barrier coat, you might need to consider other approaches.) And it didn’t scar the hull. To prevent the very sharp outside corners of the carbide blades from gouging the fiberglass, I gently rounded them with a grinder.

The carbide blades are extremely tough, but they eventually go dull. Each blade is double-edged, so when the first one dulls, you simply remove it from the scraper, flip it around, reinstall it, and scrape the next 2-foot section. I moved the tarp with me as I progressed around the boat and emptied the scrapings into a trash bag as necessary.

The key is to always use sharp, high-quality carbide blades. I later tried a scraper with non-carbide blades and made no progress. Once again, I was reminded how important it is to use the right tool for the right job.

Looking at the progress I made with six blades on that first day, I bought another dozen or so carbide blades. I tried sharpening the blades with a regular whetstone, but it did not noticeably improve the edge. You’ll need a diamond whetstone if you want to sharpen these blades.

SCRAPING TECHNIQUE

I experimented with different techniques and positions to avoid having my hands and arms raised over my head—which my rotator cuffs and shoulder bursa simply won’t tolerate. It didn’t take long to figure out the key to preventing fatigue was to keep my elbows bent and close to my side and the blade no higher than the top of my head. Then, I was able to use my body weight to pull the scraper down.

I felt very little fatigue scraping in this manner. When working near the waterline, I incorporated a short 2-foot-high step ladder to help keep my hands and arms below head level.

Having spent many hours sanding hulls and topsides over the last 25 years, it was a joy not to hear the incessant drone of a power sander for hours on end. The boatyard was quiet in March, and I could think or even listen to podcasts and music on my iPod to help pass the time.

Don’t forget to take safety precautions. Antifouling paint can be quite dangerous to handle without proper protection due to the cuprous oxide. Even though I was scraping and not sanding, I still wore a Tyvek paper suit, a 3M N95 paper mask, comfortable knee pads, lightweight gauntlet gloves, and eye protection.

CONCLUSION

There are pros and cons to the various methods described at the beginning of this article for stripping old bottom paint from the hull of your boat. For me, the best compromise between speed, physical effort required, and cost was to use a pull-scraper with a carbide blade followed by sanding.

I recommend you try a test with a scraper on your hull to see if your results are similar to mine; then decide which is the best option for you. Any method you choose will involve hard physical labor. You need to be realistic about your physical condition and be cautious about over-exerting yourself.

I averaged about three hours a day, embracing the old proverb that slow and steady wins the race. Taking on a project like this in hot weather poses an added danger of heat stroke. I highly recommend cool weather for stripping bottom paint if possible. Need I mention the importance of staying hydrated?

All that said, if you are in reasonably good shape, are capable of physical labor, and take your time, you’ll do fine. Pay attention to the temperature and humidity, use the right tools and maintain a slow, easy, steady pace, and before you know it, you are done.

RESOURCES

“Antifouling Exposure Risks,” Practical Sailor April 2022

“Too Many Layers of Bottom Paint?” Inside Practical Sailor July 2020

“Sodablasting 101,” Practical Sailor Sept 2011

“Bottom Paint Removal Decision Time,” Inside Practical Sailor June 2022

“It Takes a Tough Sailor to Make a Smooth Boat Bottom,” Practical Sailor June 2011

“Barrier Coat Breakdown,” Practical Sailor March 2009

“Boat Paint Stripper Test,” Practical Sailor March 2008