To do list of April: Let’s get to work

With the outdoor temperatures generally getting better, April is the month to put the February planning of the underwater hull into action. A step by step guide in 3 steps: preparation of the hull, the base layers and the finishing touch.


4/15/20244 min read

person hand with green and blue paint
person hand with green and blue paint

Prior to your DIY work on the hull
We touched upon the hydrodynamics underwater of keel and rudders in February. If the shape of the keel and/or rudder or even the hull does not or no longer match the original design, it is smart to have the expert reshape it prior to any antifouling application.

If the expert reshapes the keel and/or rudder outdoors or in an unheated location indoors,
consider the air temperature to be a minimum of 15 degrees Celsius and dry. The epoxy he or she uses will not give you the anticipated result and quality when it is colder.
In the preparation phase, you might repair some cracks or chips with epoxy. A no-brainer also then to consider the temperature.

Preparation of the hull
With your inspection last February, you checked the state of your antifouling and link it to your sailing plans. Racing on lakes or touring along the Mediterranean coast has a few different considerations.
In general:

When we can get away with only a soft touch, we keep the existing system and just add a layer of the existing antifouling. Mixing various antifouling is simply not a brilliant idea.
For every step in the DIY process, wearing personal protective equipment is more than a formality.

Always start with sanding. Start with grit120 and follow up with grit180 or even grit220 when racing. Wet-sanding the surface helps the antifoul to adhere well. Dry-sanding with a sanding machine without a dustbag is not recommended due to the dust it puts in the air and leaves you coughing for the rest of the day. Give some extra waterline scrub and rinse the hull with fresh water.

Make sure you put some serious effort in the preparation. The smoother the hull gets, the better the result in the following layer(s) and in the end: less resistance when sailing. Or the opposite, every irregularity and unevenness gets worse with every layer. 
And remember: be gentle with the shape of the keel and rudder! What is ruined in a minute takes a lot of effort by the pros to reshape again.

The next step is to mask off the waterline. Go for quality tape, or you’ll be sorry peeling off the residue from lesser-quality deco tape. Press the tape down thoroughly to prevent paint or dust ingress. Protect everything that doesn’t need antifouling, like anodes, skin fittings, exhaust outlets and wrap them with tape or foil.

When you change the system on the hull to another brand or want to improve it and rebuild it, call in the troops to help. Scraping off the existing layers (or what is left of it) is some serious teamwork, that one should consider at least every 6 years. Scrapers and thinner on cloth will do the trick. Once you’re done getting rid of the old layers, you can start with the sanding as explained before.

The base layers
The most common choice of DIY paints you will come across are normally either a soft antifoul, also known as self-polishing or self-eroding antifoul, and hard antifoul, often referred to as scrubbable or burnishable antifoul.

As a rule of thumb: are you racing? Use hard antifouling. As a cruiser, you might opt for the soft or erodible one. Ask a specialist who knows his trade.
What brand to use depends on a few factors in addition to your personal preference:

  • What are your plans and where? Racing? Cruising? Sea or Lakes? Are you up for many miles, or plan to leave her in the marina most of the time? How often do you plan to maintain the underwater hull? And, how much money do you want to spend on it?  

  • If design is a thing, you might want to opt for a specific colour of your antifouling; such generally restricts the choice of brands you can pick.

  • Ecologically, not all brands are allowed. Check with your local yard what brand on your shortlist they allow you to use. Often forgotten, but before you start to rebuild the new layering, always clean and degrease the hull. For a better result that lasts longer.

If you’re up for the big job and took everything off, consequently you have to start the layering from scratch: start with the primer. Check the manufacturers' guidelines for drying times and how much paint you’ll need.

The finishing touch
Next on the list: the antifouling itself. Ensure the antifoul paint is kept at room temperature to have it flow and mix well. Some even suggest having the tin in a bucket of warm water beforehand if it is too chilly. Then stir really, really, really well. Make sure you close the lid every time you pour some paint into your roller paint tray, as solvents evaporate quickly.

Best to apply the paint with a roller. Always remember: better to add 2 to 3 thin layers than 1 or 2 thick ‘n drippy ones. Again: check the manufacturers' guidelines for drying times in between the layers and how much paint you’ll need.

We haven’t touched upon the painting technique yet. For the best result, practice makes perfect. But if it’s not your day job, consider the following:

  1. Paint systematically, section by section.

  2. First a set-up the paint vertically with gentle but even pressure.

  3. Then, roll in alternating, horizontal strokes in the direction the water flows, to help conceal the strokes and more evenly apply the paint, until the area is completely coated.

  4. For some, a long roller handle helps to reach high-up areas without needing a ladder.

  5. Use a brush to paint along the waterline and everything you protected that doesn’t need antifouling prior to rolling the relevant sections.

A good tip is to mark sections on the waterline tape. Especially for the second layer and beyond, it is difficult to see what you’ve already done.

Racers using hard antifouling want to polish the hull with grit500 or even grit800. For some that means: a wet polish straight away. If you have a bit more time, a few weeks of hardening in the water allows you an even better polish, either wet or dry.

And what a good feeling when you can take all the tape off. Prior to hoisting the boat back into the water:  check the propeller and the anode, an easy task to change it while ashore.
While you’ve put a lot of effort on the underwater hull, maybe just freshen up the hull with a polish. With all the work done, you’re ready for an excellent season.

Need optimization assistance or advice? Our experts and designers have measured, shaped, and optimized many yachts, keels and rudder blades, and are ready to help you!

Photo by Amauri Mejía on Unsplash