Perhaps one of the worst jobs in boat ownership is bottom renewal. Every 20 years or so, you really ought to strip all the old bottom paint off and start again.  And while you’re at it, you should apply a new barrier coat – particularly if you are planning to have the boat spend long times in warm, salt water.

Fiberglass gelcoat itself is not entirely waterproof, and the bottom paint that sits on top of the gelcoat is not even remotely waterproof.  So, with age, boats become susceptible to blistering.  And if your boat’s hull has balsa coring (as we do), them the problem can get very, very nasty indeed.

Last year, Kath and I attended a series of excellent seminars hosted by The Rigging Shoppe.  Among the series was an excellent half-day session on fiberglass upkeep and repair, presented by Rod Brandon of Sheen Marine.  Rod told us all about a nasty process called “osmosis” and how it led to a problem of blistering on the hull.

It turns out that the risks of suffering from osmosis problems are measured much like the risks for diabetes or heart disease.

Is there a family history? When it comes to laying up a fiberglass hull, technique matters.  Some builders took more care, and used better techniques/materials.  Over the years, it becomes clearer which builders did the best work.  Now, by speaking with owners of boats from the same builder, you can get a sense of whether or not your boat is likely to have the problem in the future.

Are there lifestyle risks?  If it’s too cold for swimming, it’s good news for your hull.  As with many chemical processes, temperature accelerates the process of osmosis that leads to blistering.  Similarly, salt water is far more likely to lead to blistering than fresh water.  So, a season or two in the Bahamas can take a toll on a boat that happily spent decades in the cold, fresh waters of the great lakes.

Luckily for us, Hinterhoeller boats report few incidence of blistering.  However, we think of ourselves as Spartan’s curators.  She’ll be turning 40 in a couple years, and our goal is to keep her in nearly new condition throughout her time with us.  So, we took the plunge and set to work.

Time scraping or sanding: 35hrs
6″ Sanding Disks: 78
Scraper Blades: 14
Pounds of paint dust collected: 30lbs
Shopvac filters: 3
Tyvek suits: 4
Earplugs: 8
Trips to Home Depot: 5
Days of arm/back pain: 5

Things Learned

Scrape First – There’s no point spinning up a sander while there’s 20 years of paint amassed against the hull.  Your sandpaper will be gummed up in seconds and you’ll be spitting out squalls of toxic dust.  You need to start by scraping most of the paint off.

Strippers Aren’t Miracle Workers – Strippers seem to work well on some spots and not at others.  If you’re working in a cold climate, they might not work at all.  And even when they do work, you’ve still got lots of scraping to do.  So go ahead and experiment with them if you have time, but don’t get your hopes up until you see results.  We tried leaving strippers on for 1, 3, 5 and 24 hours, with and without a plastic covering.  There was little difference in the results and the gummy paint was really no easier to take off than the hard paint which often came off in large chips.

Use Carbide Scrapers – Don’t bother with traditional pull scrapers.  You’ll spend half your time sharpening the blades.  Use a scraper with a carbide blade.  Buy some extra blades.  Switch the blade every hour or so.

Change Sanding Discs Often – You’re not going to save enough money to warrant the extra time you’ll need if you try and spare the sanding discs.  Find a good bulk deal and get at least 50 discs.  Stop every 10 minutes or so and replace the disc.

Careful on those corners – In the rounded spaces where the keel and hull met, I made more than a few mistakes, letting the edge of the tool dig into the glass under the gelcoat.  A light touch with a constantly moving sander goes a long way to limiting the patching work.