How to apply hard antifouling to the underwater hull

In this article we explain how to protect the below waterline part of the boats hull with a hard antifouling from the brand International. Here we used International Interspeed Extra. This popular antifouling system is suitable for use in fresh, brackish and salt water.

Questions about antifouling? Feel free to contact our specialists!

What is hard antifouling?

Hard antifouling paint eliminates fouling caused by aquatic fauna and flora. It protects your underwater hull from the growth of algae, mussels and small organisms. Thanks to a hard antifouling system, the sailing characteristics remain optimal and you prolong the life of your boat. Hard antifouling contains biocides that are slowly released when they come into contact with water. Hard antifouling is especially suitable for fast-sailing vessels (from 25 knots) because self-sharpening antifouling washes away too quickly at high cruising speeds.

After the boating season a hard paint layer remains. Therefore you build up a hard paint layer over the years, which has to be removed after a few years before you can apply a new layer. Hard antifouling can be therefore be used for several years and can be polished in between.

Want to buy hard antifouling?

At Polyestershoppen, you can shop various top quality hard antifouling systems that will keep fouling at bay throughout the season. We supply antifouling coatings by International and Hempel from stock at lightning speed so you can get to work quickly.

Curious about our complete antifouling range? Take a look at the other hard antifouling systems.

How to apply hard antifouling [manual]

In the video below product expert Arne shows you how to apply International hard antifouling. We wrote down all the steps so you can read it again later.

Step 1. Prepare the surface

First of all, make sure the underwater hull is in good condition. Repair cracks and holes, treat osmosis if necessary and fair the hull. Then pre-treat the surface properly with the right primer:

• Old self-sharpening antifouling:
  Remove the old self-sharpening antifouling with a pressure washer or vacuum scraper.
  
  
• Old hard antifouling:
  Remove the old hard antifouling with a pressure washer or a vacuum scraper. 
  
  
• Unknown antifouling:
  Remove the unknown antifouling and apply International Primocon as a primer.
  
  
• Bare polyester substrate:
  Pre-treat a bare polyester substrate with International Gelshield 200 (osmosis protection) or International Primocon.   
  
  
• Steel/wood/lead substrate:
  Pre-treat the surface with HB Coating, International Interprotect or International Primocon primer.
  
  
• Aluminum substrate:
  Pre-treat the surface with International Interprotect or International Primocon primer.

Degrease the substrate with Double Coat degreaser and apply primer as required.

Step 2. Apply antifouling

Required products

• International Interspeed Extra antifouling in the desired colour
• Nitrile/latex gloves
• Velour rollers and paint brackets
• Flat brushes
• Mixing spatulas
• Paint trays

Mark (imaginatively) the underwater hull in different parts, such as port front, port rear, keel, rudder, etc. Estimate the size of each part and calculate how much antifouling you need. Count on a yield of approximately 10 m2 per litre of antifouling.

Pour the required amount of antifoulant into a mixing cup and then into a roller tray. Now apply the antifouling evenly with a brush or velour roller. By estimating the size of the surfaces and by measuring, you keep an eye on whether you are applying too much or too little antifouling.

Step 3. Apply a second coat of antifouling to heavily stressed parts 

After approximately 6 hours (at 23°C), the antifouling can be repainted. Please check International Interspeed Extra's technical documentation for the exact repainting time per temperature. Apply a second coat of antifouling to the bow, keel, rudder and just beneath the waterline. After this second layer of antifouling has dried properly, the boat can be launched.

You can optionally sand the antifouling to reduce water resistance.

Step 4. Follow-up 

The underwater hull has now been coated with a hard antifouling system and you can enjoy your boat to the full for 2-3 seasons. After this period of time, remove the layers of hard antifouling again with a vacuum scraper and repeat the above steps.

Are you going to refurbish the hull above the waterline? Then check out our information article on boat varnish!

More information on hard antifouling?

Do you have any questions about International Interspeed Extra or another hard antifouling? Feel free to contact one of our specialists. We are happy to advise you to help make your project a success. Send us a WhatsApp message, send an email to support@polyestershoppen.com or call +31 85 022 0090 for free, no-obligation advice.