Making a Case for Sodablasting Your Hull

Posted by Darrell Nicholson at 11:50AM - Comments: (11)

September 27, 2011

Quick. What’s your least favorite boat maintenance project? Cleaning the bilge? Changing the engine oil?  ... How about stripping off several years worth of bottom paint?

One of the first things my wife, Theresa, and I did with our cruising boat was to strip 10 (20? 30?) years of bottom paint off of the old wooden ketch using an 8-inch electric sander. It probably shaved 10 (20? 30?) years off of my life. I wince just thinking about that summer.

A couple of seasons ago, PS Technical Editor Ralph Naranjo and contributor Frank Lanier took a different approach: chemical strippers. These reduced the amount of sanding involved, sparing some backbreaking work, but that was about it.

After that experience, Ralph decided to look into sodablasting, featured in the October 2011 issue of Practical Sailor. One of the chief complaints you hear about any for-hire boat work is the exorbitant price charged, but once you start to do the math—and start thinking about your health—a $1,500 fore-hire sodablasting job doesn’t seem so indulgent. 

Stacey Stone (left) of Chesapeake Blasting services and trainees go through details of setting up a boat hull for sodablasting.

One of the biggest mistakes an owner makes when estimating how much time it takes to strip a hull is to peck away at one of the easy spots where the paint is peeling and then assume the rest of the coating will come off just as easily. Ralph gives a more realistic formula for estimating the amount of time a stripping project will take. First, you estimate the wetted surface area (WSA) using the formula: waterline length (Lwl) x (beam waterline (Bwl) + draft (T)). Multiply this number by .75 for a medium-displacement boat or by .5 for a light-displacement boat. This gives you a very crude estimate of surface area, but it’s fine for estimating paint removal (or painting).

Now that you’ve got your total area, you can figure out the amount of actual time it will take you to do the job. Start your stopwatch and attack one square-foot of an “easy” section. Do the same to a patch where the paint is well adhered. Now estimate how much of your hull will be “easy” to strip and how much will be hard. No cheating! It will only bite you in the end. Once you’ve estimated the percentage of hard and easy sections, you can apply your timed results to the total area of the hull.

Here’s an example: Your boat has a 30-foot waterline, a 6-foot draft, a waterline beam of 10 feet, and is a medium-displacement vessel. Our fuzzy math for a medium-displacement sailboat  

WSA = Lwl x (Bwl + T)

says you’ve got 360 square feet of paint to strip: 30 x (10 + 6) (.75) = 360.

Next comes the all-important apportionment of “easy” versus difficult paint removal. In this case, 85 percent of the hull is tough stuff, taking four minutes per square foot to strip: 0.85 x 360 x 4 = 1,224 minutes of backbreaking work. Add in the 15 percent of easy scraping area that takes only 30 seconds per square foot to strip: 0.15 x 360 x 0.5 = 27 minutes. The result is 1,251 minutes, or just under 21 hours of serious scraping. That’s 21 hours of serious work, not including coffee breaks, admiring sisterships hauled out in the same yard, or gazing numbly at the water wishing you were sailing.

Now, how much is your time worth? And don’t forget the money you’ll be spending on scrapers, chemical strippers (if you use them), sand paper, etc. As much as I like to do my own boat work, this is one for-hire job that is worth considering.

Comments (11)

Regarding Kent's insightful comments on chisels and scrapers. We have touched on this approach before, and I should have mentioned it again here.

There's a pretty good discussion of the handiness (and hazards) of using chisels for bottom paint removal in our March 2009 article:

http://www.practical-sailor.com/issues/35_3/features/Epoxy-Barrier-Coat-Removal_5749-1.html (Click on the link in the right-hand box called "Tools and Techniques.")

We have another article on removing varnish using custom-made scrapers. Custom-made scrapers were essential to our refit of the Cape Dory 25, used mostly for removing varnish.

http://www.practical-sailor.com/issues/36_3/boatreview/Cape-Dory-Boat-Maintenance_5896-1.html

Sharpness is a key, and maintaining a sharp blade is an art form in itself. Several woodworking websites deal with this topic.

Posted by: DARRELL N | October 5, 2011 1:59 PM    Report this comment

I have stripped 4 cruising boats from 34 to 45 ft in the last 25 years and I am amazed that even PS (which is usually a great source for accurate advice) ignores the best way to strip a bottom. Sanding and chemicals are dirty, toxic, and expensive. My method is to use 4 high quality wood chisels (2', 1 1/2", 1" & 1/2") and a sharpening stone. You start with the 2" blade on the loose areas with the blade bevel out and at a very oblique angle. In one swipe (2 sec) I can take off a 2" by 18" strip (1/3rd sq ft).. When the 2" blade stalls on a tough spot I drop to the 1 1/2' and then the 1" and then the 1/2 inch. You sharpen the chisels frequently! It takes some practice to not gouge the gel coat but this gets 90% of paint off down to bare glass. which comes off in chips (not toxic dust or glop) and is easy to collect on a tarp. The last 10% (turn of the bilge, round of the keel etc.) I use stripper and or sanding. It took me and a helper less that 16 hrs to do my Bristol 45.5 (approx 2,400 sq ft per the PS formula). The sandblasting quote was $4,000! It is tough on the shoulder muscles but good exercise. I was 59 when I did the Bristol 6 yeras ago.

Posted by: KENT B | October 1, 2011 2:41 AM    Report this comment

While on the topic of sensible ways of removing antifouling from FRG hulls, has anyone any experience with removal by dry ice blasting? The aviation world has used dry ice blasting to successfully remove paint from aluminum fuselage skins, without the many environmental concerns associated with chemical strippers, and the somewhat delicate surface is not damaged. I'd be interested in cost comparisons...

Posted by: WM | September 29, 2011 11:44 AM    Report this comment

My 42 foot sailboat with wing keel is $2k. At an estimated 20 hours to chem strip and $700 of peal away, my per hour wages to strip would be $10 per hour plus the $700 stripper. Lets not talk about the lost time. Guess I am soda blasting

Posted by: Catalina 42 | September 28, 2011 6:37 PM    Report this comment

Hope you have more spare time than I do. Considering removal of old hard paint (many years) on a 28 footer and some blister repair 3 coats of barier 3 coats of epoxy and 4 coats of anti-foul it only cost me one summer. PS

Posted by: PHIL S | September 28, 2011 4:50 PM    Report this comment

And we wonder why our five year old is struggling with math: Here is Ralph's original formula:

WSA = Lwl x (Bwl + T)

For more info on soda-blasting check out the article or call the folks at Chesapeake Blasting Services.

Posted by: DARRELL N | September 28, 2011 3:26 PM    Report this comment

Something else to consider... Any bottom paint that is above the waterline will be hard as rocks and careful scraping along the boot stripe is very tedious. This adds huge chunks of time not to mention moving the stands or cradle, scraping those areas, under the keel etc.

Posted by: Jay K | September 28, 2011 3:23 PM    Report this comment

I've had a couple of older boats that I've attempted to strip physically and chemically, using several different methods of the latter. I guess it all depends on how much you value your time, knuckles, weekends, and nights out (without falling into your plate). That's why I had my last boat soda blasted. It was worth every darn nickel.

Posted by: ED W | September 28, 2011 2:42 PM    Report this comment

OK article but something is amiss with the formula. In the body of the article you state: multiple LWL x Bwl x draft. But when you do the calculation for the example you have added BWL plus draft then multiplied by LWL and then applied the .75 factor.

With the curve of the boat I'm not sure how the formula was arrived at but I'm guessing that 360 is closer then 1,350 (30x10x6=1800x.75=1350). Which is it?

fb

Posted by: FRANK B | September 28, 2011 2:31 PM    Report this comment

You sold me on not sanding and scraping so... any more information on the soda blasting option?

Posted by: ANDREW N | September 28, 2011 2:20 PM    Report this comment

When we purchased our 33' sailboat boat in 1987 she already had 13 years of accumulated bottom paint. We chemically stripped the bottom paint over the course of a couple spring weekends, and repainted with an ablative paint. More recently during the spring of 2009 I decided I wanted to strip the bottom, apply a barrier and repaint. After making a calculation similar to the one in the article I chose to have the bottom soda blasted. I'm glad I made that decision. The work of barrier coating and painting was more than enough for that spring.

Posted by: GUY J | September 28, 2011 2:19 PM    Report this comment


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