Fouling occurs in all water environments throughout the world. Marine algae grows on any surface when prolonged to long periods underwater. And once you have a slimy layer of algae, then comes the growth, weed, barnacles etc.. So the trick is to stop the build up of algae before any growth can take hold.

Why antifoul my boat?

Fouling, or growth, on a boat hull will cause all sorts of performance problems from reduced speed, increased fuel consumption and can even affect your steerage. While a slower displacement sailing boat might feel the effects of growth less, a faster planing boat might simply fail to achieve its planing speed.

Painting your boat with antifouling paint is probably the most common defence against growth although there are other solutions, some of which I’ll mention later.

Do I need to antifoul my boat?

It’s worth giving some thought to whether you want to antifoul your boat, as once painted it’s very hard to remove. For example, I often get asked about antifouling small trailer boats that might just sit on a mooring or marina for a few months of the year.

Well that decision has to be yours, but my advice is only paint your hull if you really need to. If for example, you’re leaving her the water for a couple of months in the summer then it’s probably not worth it. Just pull her out and have her jet washed every month. Or do it yourself every two to three weeks. This will probably save you money and time in the long run.

But if your boat is to sit on berth or mooring all year round, then antifouling is probably a necessity. And if you decide that anti-fouling your boat is the right choice, then this guide will explain all you need to know.

One piece of advice that might help any decision is that most paint manufacturers require the boat to be put in the water within a couple weeks of painting else the active biocide properties within the paint become mostly ineffective.

What paint should I buy?

I’m afraid there’s no ‘one peg fits all’ paint. The amount of fouling, and the type of fouling vary considerably depending on location, water temperature, amount of sunlight, nutrients and salinity. It’s often a good start to ask others in your chosen location and find out what works for them, and go from there.

Antifoul paints by their very nature allow gradual erosion of the paint layers, this keeps fresher active biocide on the paint surface allowing it to work more effectually. However, fast or planing boats will need a ‘hard racing’ paint that’s designed to erode less, else it would simply dissolve in the water when the boat is speeding along.

Most manufacturers will make a paint designed for your type of boating. However, if you are in any doubt, just ask the paint seller for advice.

Preparation for a non painted hull

If you’ve never painted your hull before, there are a few extra steps to consider.

Probably the most important first step is to work out where your waterline is so you know where to paint up to. The simplest way to do this is to leave the boat in the water for a about week or two.

Store everything away in its right place, fill the tank with fuel, fill any other containers such as water tanks, and make sure she’s sitting naturally level, especially abeam level.

On pulling her out, you should notice a distinct tide mark all around the hull – use this as your waterline. Mark off with masking tape about an inch higher than this mark. When masking around your outdrive or outboard mounting, mask in a way so that you leave an inch of exposed hull between the finished antifoul paint and the drive mounting. This is a necessary precaution to prevent any electrolysis.

So you’ve established your waterline and have masked off the area to paint. You’ll now need to give the whole area below the waterline mark a sand with medium grade paper to provide a key for the undercoat to stick to. It’s very important you have a smooth finish, so use this opportunity to fill in any dings and scratches with suitable filler, sand and smooth off. Once you’re happy then wash the hull to remove any traces of dirt, grit and sanding residue. I would wash with warm soapy water twice, then dry with a cloth, and allow to air dry for at least a couple of days. It’s very important you end up with a smooth and totally dry base before you start painting the hull.

Preparation for a previously painted hull

It’s usual to have your hull jet washed clean as soon as your boat leaves the water thus removing far more growth than if jet washed when dry. You may have to manually scrub, scrape and clean off any tough areas of growth to fully prepare your hull. Once your hull is as clean as you can possibly get it then I would suggest letting it dry for a few days.

You can mask your boat based on the existing waterline left by the previous painting, not forgetting to leave a good inch of exposed hull around the outdrive or outboard mounting. Now you can sand with a medium grade paper to provide a key for the undercoat, but also to smooth out the many imperfections left by the previous paint. Some do this hand, but it’s quite common to remove much of the existing paint with an orbital sander. Any dings and scratches need to filled with a suitable filler, sanded and smoothed off. Once you’re happy with the finish then wash the hull to remove any traces of dirt, grit or sanding residue. I would wash with warm soapy water twice, then dry with a cloth, and allow to air dry for at least a week. It’s very important you end up with a smooth and totally dry base before you continue to painting the hull.

Painting the hull

Make sure you adhere to the instructions and guidelines for any paint you buy and adjust our advice accordingly.

So you should now have a clean, smooth and ultra dry surface all ready to paint. The importance of a smooth clean surface can’t be expressed enough here. Any kind of finish is always proportional to the amount of preparation done – there are no shortcuts…

Most paints are toxic so make sure you wear rubber gloves and eye protection, and a set of clothes that you don’t mind discarding when you’ve finished. So assuming you’ve decided on your brand of paint, you can now get started. It’s more than likely you’ll need to apply a good primer or undercoat first, but not always…

  • If your boat has never been previously painted then you should apply two coats of suitable primer.
  • If your boat has been previously painted then you can often get away with one coat, and sometimes no primer at all. However, any areas that have been filled or sanded back to the hull will require primer.

Once your primer has dried sand over with light paper, wash with warm soapy water and allow to dry. You can then apply on your antifoul paint. Apply two medium coats making sure there’s no running or bubbles, remove any brush hairs, flies or other obvious particulates that find their way into the finished surface.

Where to paint your boat?

Most will choose to block off their boat in a yard so access is generally good. The main keel blocks take the weight of the boat while the side jack stands stop it from falling over. Although you’ll want to relocated the jack stands to fully paint the hull, most boat yards will not allow you do this for H&S reasons, so you’ll have to enlist their help. Assuming you boat has been blocked off via a lift, then you can finish the sections restricted by the keel blocks once the boat has been safely lifted and secured just prior to going back into the water.

You can however also paint your boat on a trailer. Access is obviously more restrictive but quite possible with some patience and perhaps a little ingenuity. If on a roller trailer, you can usually and safely move the boat back and forth a few inches to expose the areas covered by the rollers.

If on a bunked trailer, and assuming your boat is resting on keel rollers, then you can use a couple of jack stands or build temporary side supports allowing you to drop one side of your bunks to paint behind them. The weight of your boat is usually supported by the keel rollers, the bunks in effect stop the boat from rolling over. You can also get to the lower sections under the keel rollers by temporarily jacking up your boat by an inch or two. Just remember to be very careful, enlist some help, and perhaps tie off the other side of the boat to a tow bar or even a large tree for extra safety.

As mentioned above, there are other solutions to painting your boat, and the most common are boat lifts. We have a separate article on boat lifts here.

Disclaimer

Please note Marine Advisor offers this information in good faith but takes no responsibility for any adverse effects, injuries or damage that may result in following our guide. Working on boats can be dangerous and antifoul paints are usually toxic. Please take any necessary precautions, enlist help and get advice when needed.

Thank you,
Marine Advisor Team