Untreated wood absorbs water, rots and grows marine organisms, so a wooden hull is protected with pine tar or Stockholm tar (heated and brushed on in several coats), linseed oil (several coats over days), or oil-based paint over a primer, which the carrier calls the best all-around protection, with a copper-based or traditional tallow-and-sulphur coating below the waterline against barnacles and weed. Find the quantity by coating a measured test patch: if 0.1 L covers 0.25 m², one litre covers 2.5 m² and the 8.1 m² of outside of a 4.5 m hull needs about 3.24 L per coat.
Part of Canoe & Rowing Boat Construction
Surface treatment is the paint, oil or tar applied to a boat’s wood to keep out water and, below the waterline, to deter fouling.
The Coat Has To Match What The Boat Meets
Wood rots when it stays wet and cannot dry, and weed and barnacles attach to a hull that sits in the water for weeks. The carrier lists four treatments: pine tar or Stockholm tar, the traditional wood-boat treatment, applied hot with a brush in multiple coats; linseed oil, which penetrates the wood and gives moderate water resistance, in multiple coats over several days; oil-based paint over a primer; and a bottom coating below the waterline. Historically boats were coated with tallow mixed with sulphur, or had copper sheets nailed to the bottom, according to the carrier.
Do not mix systems carelessly. Tar bleeds through paint, and paint applied over wet wood peels and traps moisture. Pick one system for the topsides and one for the bottom, and apply each to dry, clean wood. Boat Sealing covers pitch coats and the preparation of the hull surface.
What You Need
- Your coating: tar (see Pine Tar), linseed oil, oil paint and primer, antifouling
- Brushes, or cloth swabs on sticks
- Clean, dry wood: sanded or scraped to bare wood
- A dry, warm day and a place to work away from flames
- A test board of the same wood, 0.5 m × 0.5 m
Method: Coat In A Fixed Order
Step 1: Prepare the surface. Scrape and sand; wood must be dry (see Moisture Content).
Step 2: Measure coverage. Brush a measured volume on the test board (say 0.1 L on 0.25 m², an assumed example) and compute litres per square metre.
Step 3: Prime or tar the whole hull, inside and out, with the end grain and the stem and keel joints getting extra.
Step 4: Apply further coats after each has dried or soaked in, the pause set by the product: linseed oil several days between coats (the carrier says “over several days”); tar, hot; paint, as it dries.
Step 5: Coat the bottom with antifouling if available.
Worked Example: How Much Do You Need?
Hull outside area, approximated as total girth × length: if each side measures 0.9 m from keel to gunwale (the planking assumption), the total girth is 2 × 0.9 = 1.8 m, and the area is 1.8 × 4.5 = 8.1 m². This ignores the curvature and the ends, so allow 10-20% more (an assumed margin). From the test patch, 0.1 L ÷ 0.25 m² means 2.5 m² per litre, so one coat on the outside needs 8.1 ÷ 2.5 = 3.24 L. Three coats need 9.7 L, and the inside and a bottom coat are extra. Use your own test patch, since coverage differs widely between tar, oil and paint.
Common Mistakes
- Painting over wet wood — it traps moisture and rots from inside.
- Painting over tar — tar bleeds through.
- A single heavy coat — thin coats penetrate better than one thick one.
- Skipping the end grain — end grain absorbs the most water.
Safety
Fire, heat and copper
Linseed-oil rags can ignite on their own as the oil cures, and hot tar burns skin and ignites; hang rags spread flat and keep tar away from flames (see Oiled Cloth & Animal Membrane). Sulphur mixtures are flammable. Copper-based antifouling is toxic to water life and to skin; wear gloves and do not apply it near a drinking-water source.
What’s Next
- Learn next: Oars & Rowing Technique — putting the finished hull to work
- Combine with: Caulking & Waterproofing — the seams under the coating