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Antifouling is one of the least pleasant boat maintenance jobs to do, but it has to be done. The very worst job of all is removing the old antifouling as this can get seriously messy and is very hard work.

The easiest way to reduce the build up of antifouling is by using eroding antifouling and to make sure that the hull is always pressure-washed as soon as it comes out of the water. Much of the remaining antifouling will be removed this way.

Health and safety precautions

As antifouling is toxic and messy stuff, you must use suitable protective clothing. The biocides in antifouling can be absorbed through the skin so you need to protect yourself against this as well as any dust particles that you could breathe in. Wear a good quality breathing mask that protects you from solvents, also rubber gloves, safety goggles and overalls. I normally wear an old hat for additional protection.

Lay down appropriate sheeting beneath the hull to collect any old paint and residue. You should check with your boatyard what the local regulations stipulate about removing and applying antifouling. You may need to use suitable tenting to protect others nearby.

Antifouling in good condition

If the existing antifouling surface is even and there are few or no imperfections, then you can safely say it is in good condition. This means that preparing the hull before applying new antifouling will be comparatively easy whether you opt to go down the DIY route or ask professionals to do the antifouling for you.

Preparation

Use 80-grit wet and dry abrasive paper to sand back the old antifouling. Be careful not to abrade too much as you want to keep the primer coat beneath intact. If in doubt, use 120-grit paper as it will be less abrasive. You need to keep the paper wet, so have a bucket of fresh water beside you to rinse in as you work.

When you have finished sanding, thoroughly rinse the surface with fresh water and allow to dry before applying the antifouling.

Antifouling in poor condition

If the surface is uneven and cratered, caused by patches of old paint that have flaked away and been progressively overpainted, then it is time to get your boat’s bottom back into shape. All the old antifouling layers need to be completely removed before new antifouling can be applied.

You may decide to employ a professional to do all this work for you and if you have the budget for this it will be money very well spent. Although this is the expensive option, it will save you considerable time, hassle and back pain. Remember also that applying antifouling in future seasons will be much more easy going.

Removing old antifouling

There are a number of options available for removing layers of old antifouling:

Option 1: Scraping method

For those boat owners choosing the DIY route, scraping is the budget option. This method is also the most arduous, taking a lot of time and considerable effort. I have done it quite a few times. The last time I did so I promised myself that I wouldn’t do it again. It can be ok for an hour or two but the novelty soon wears off and you need to be prepared to be bent over double for a few days to scrape a whole hull. The only compensation here is that you are not paying someone else to do the work. Otherwise there is zero satisfaction gained from doing this particular boat job.

There are various types of scraper to choose from – flat-bladed types which you push, triangular-shaped ones that you pull and double-handed ones with very sharp carbide scrapers that you also pull towards you. You need to keep the blades sharp but at the same time you have to be careful they do not dig into the gelcoat which can leave a nasty gash that you will need to fill. It helps to round off the edges of the blades to avoid such mishaps.

Avoid scraping old antifouling on a windy day as the scrapings can be blown all over the place. It is best to collect them up as you go into a refuse bag.

Eventually you will reach the point when you decide you cannot do any more scraping. If you have any energy left, you will then need to wet sand the whole surface with 80-grit paper and rinse it off ready for painting. Alternatively, stagger home and come back the next day to do the sanding.

Option 2: Chemical stripper

There are several types of paint stripper available which are specially formulated for the removal of old antifouling. These products can make antifouling removal easier as they soften the old paint, making it less labour intensive. However, this method increases the amount of toxic waste to dispose of and can get very messy. If you are removing several layers then you may have to repeat the process two or three times. Some paint stripping chemicals are more environmentally friendly than others and biodegradable, so it is worth shopping around.

Your boatyard or local chandlery should be able to advise you on which product to choose for your boat. Make sure the antifouling stripper you choose will not harm gelcoat if your boat has a GRP hull.

Most antifouling strippers can be applied with an old brush, using a stippling action to help the stripper work through the old layers. It is a good idea to then cover the treated area with clingfilm to prevent the stripper from drying out as it slowly softens the old paint. Some chemical strippers are faster acting than others so it pays to check before you buy.

After the stripper has done its job, the old antifouling can be removed with a wooden or plastic scraper while the old paint is still soft.

You will then need to wet sand. If the primer is still intact then rinse the hull with fresh water, leave it to dry and it will be ready for painting.

Option 3: Blasting

There are a number of methods used for blasting a boat’s hull to remove antifouling. These include using either dry ice, sand or soda. They do require costly equipment and usually a team of two or more people to do the work, which will include tenting off the boat to protect everything else in the immediate vicinity.

A compressor is needed, so for DIY purposes this method of paint removal is really a non-starter unless you can hire suitable equipment and have some assistance to do the work, as well as the permission from your boatyard to use a blaster.

It will pay to ask around for the most cost effective method for your hull. Soda blasting is definitely worth investigating. It is a ‘non-abrasive’ blasting which uses a type of sodium bicarbonate (baking soda). It does no damage to sub-surfaces, so soft materials like aluminium and timber can be blasted without damage. Soda crystals exit a jet nozzle at high speed and as each crystal explodes on impact, the sudden release of energy blasts away the coating on whatever is being blasted.

Soda is harmless to the environment and washes away with water. However, soda blasting is normally done by tenting the blasting area using plastic sheeting to form a tent and more sheeting on the ground to catch the old antifouling and residue.

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