Choosing boat bottom paint: a guide for inland waters and the sea
Lauri Koski
Choose boat bottom paint according to the water, hull material and existing paint. Priming and controlling fouling serve different purposes.
What you need for the chosen job
- Hempel Pre-Clean and rinsing water.
- Preparation and painting tools, personal protective equipment and protection for the work area.
- Hempel Light Primer when the substrate needs epoxy priming as part of the system.
- Your chosen bottom coating and its intermediate coats. You do not need every product in this guide for one system.
Start with three questions
Where is the boat used, what is the hull made of, and what is on the bottom now? Identify the old paint name or type before buying. Do not build a new system over flaking paint. Also say whether the boat stays afloat all season or is frequently lifted onto a trailer.
Biocide-free coating or conventional antifouling?
Conventional biocidal antifouling uses active substances to control growth. A biocide-free system relies on the physical properties of its surface. Biocide-free products also differ in their finish, preparation and maintenance requirements.
Biocidal antifouling is prohibited in inland waters – and is unnecessary there. In Finland, biocidal antifouling paints must not be used in fresh water, including lakes and other inland waters. The bottom can be kept clean, for example, by washing it and lifting the boat ashore. Where needed, choose a biocide-free coating suited to the use. This does not ban all primers: the hull’s priming and protection needs are assessed separately.
A biocidal product used at sea must be approved for the intended use, and its label restrictions must be followed. Biocide-free coatings, bottom cleaning and keeping the boat ashore can also be alternatives at sea.
Choose for the way you use the boat
| Boat use | What to compare |
|---|---|
| Lakes and inland waters | A biocide-free coating and a cleaning method; no biocidal antifouling. |
| Long periods afloat at sea | A biocide-free system or antifouling approved for the intended use; consider access for cleaning. |
| Trailer boat or frequent lifting | Our shop recommends the Hempaspeed TF route, not Silic One. Check the substrate and existing coating. |
| Aluminium boat | A complete system explicitly suitable for aluminium and immersion. |
Do not choose copper-containing antifouling for aluminium solely by colour or price. The topcoat manufacturer must permit its use on aluminium; an isolating primer is not blanket permission to use any topcoat.
Primer protects the substrate – the finish helps keep it clean
An epoxy primer prepares the substrate and builds the protective layer specified for the system. Antifouling or a biocide-free finish does not replace it. A one-component adhesion primer such as Underwater Primer does not serve the same purpose as an epoxy moisture barrier. Check coat numbers and dry film thickness in the chosen system's instructions.
Silic One or Hempaspeed TF?
Hempel Silic One is a soft silicone coating whose smooth surface makes it harder for growth to attach. Hempel Hempaspeed TF is a hard, biocide-free thin-film coating. Both may need cleaning. Choose according to use and maintenance; neither removes the need for proper priming.
What happens to the old paint?
A sound, identified paint surface can sometimes be retained with an approved intermediate coat. Investigate unknown, soft or loose coatings before painting. Conversion Primer does not rescue poor adhesion and is not applied directly to a bare hull.
- New suitable substrate, Hempaspeed TF: material-specific preparation → epoxy priming specified for the system → Conversion Primer → Hempaspeed TF.
- New suitable substrate, Silic One: material-specific preparation → epoxy priming specified for the system → Silic One Tiecoat → Silic One.
- Sound, suitable old antifouling, switching to Silic One: verify adhesion and compatibility → cleaning and preparation → Conversion Primer → Silic One Tiecoat → Silic One.
These are different starting conditions, not a list of all the primers to put on one surface. Maintain sound Silic One according to its own instructions. Exact coats, overcoating intervals and launching times depend on the product, temperature and data sheet for your paint batch.
Paint quantity, roller or brush?
Theoretical litres required equal area × coat count ÷ spreading rate. Calculate primer, intermediate coat and finish separately, allowing for losses caused by the surface and application method. Select colour and pack size on the product page. A roller is quick on large bottom areas; a brush helps around edges and tight spots. Follow the product's tool and thinning instructions. Do not replace paint quantity by adding more thinner.
Protect the work area and collect paint and sanding waste. Do not wash biocidal antifouling while the boat is in the water. Wash and degrease before sanding so that contaminants are not spread into the substrate.
Find products and help with choosing
Antifouling and bottom coatings · Primers · Hempel · Epifanes.
A brand or product name alone does not establish suitability below the waterline. If the starting condition is unclear, use VeneMaaliGURU (in Finnish) or email the hull material, existing coating and use to info@venemaalit.fi.
Continue to the job-specific guides
- Painting a boat bottom: a guide for fibreglass and aluminium boats
- Varnishing a wooden boat: an even gloss finish and choosing 1K or 2K
- Switching to a biocide-free bottom coating: Hempel Silic One guide
- Boat osmosis: identifying blisters and planning an epoxy repair
- Repairing boat scratches and painting above the waterline
Preparing the boat surface before painting
Manufacturer sources and use restrictions
- Hempel – Hempaspeed TF
- Hempel – Silic One
- Hempel – Light Primer
- Hempel – Conversion Primer
- Tukes – antifouling
Updated 13 November 2026. Check the current data sheet for your paint batch and the use and safety instructions on the packaging.
Need help choosing products and a coating system? Open VenemaaliGURU (in Finnish) →