Preparing to Paint

Why Do I Need To Paint My Boat / Preparing To Paint / Painting Advice And Specifications / Painting Substrates / Osmosis




Planning The Job

General considerations:

• Consider the total process, including surface preparation and paint application.• Decide on the type of paints you’re going to use. • Consider drying, curing and recoating intervals. • Consider anticipated temperatures and weather conditions.

Indoor and outdoor painting advice:

• If you’re painting indoors, make sure there’s plenty of ventilation so the paint solvents evaporate and the paint cures properly. • If you’re painting outside, pick a calm weather day. This minimises the risk of dust pollution on the paint surface, and allows solvent based paints to flow out naturally, which will improve the final finish.

Temperature and humidity:

Our paints are very tolerant to varying conditions. The drying/curing and  re-coating times we give in this guide are (unless otherwise stated) based on:

• temperatures of 10oC and 20oC

• relative humidity (RH) of 60–65%

• a well ventilated working area.


You may need to adjust these drying/curing times depending on the conditions. As a guide:

• double drying/curing times for a drop of 10oC

• halve drying/curing times for a rise of 10oC

• adjust accordingly between these temperatures.

Remember that paint properties change with temperature variation. At lower temperatures, paint thickens so may need thinning. Always note correct/maximum thinner ratios, and take care not to add more than is recommended. Avoid painting above recommended maximum temperatures, as the faster drying/curing rates reduce the paint’s flowing properties, and this can result in visible application marks. This can also happen if you paint in direct sunlight, where the surface has a much higher temperature than the surrounding (ambient) temperature.Check minimum application temperature of the paint you are using as the paint will not cure below it, resulting in poor film formation, poor adhesion between coats and unsatisfactory gloss finishes.

Relative humidity

Ideally, relative humidity should not be above 65%. (You can measure humidity with a hygrometer.) A good test is to moisten the surface to be painted. If it dries within 10–15 minutes it should be okay to paint.

Key Advice: Do not paint outdoors too early or too late in the day, when there’s a risk of condensation or dew.


Personal protection

Ensure you wear suitable protective clothing, including gloves and glasses. Read labels carefully and follow all application and health & safety advice. Open cans with care. Don’t eat or drink in the vicinity of stored or applied paint. See the Health and Safety section for more detailed guidelines


Surface preparation:

  1. Removing old paint and antifouling

Removing old paints and antifoulings can be easier with Paint Stripper, which is a highly effective solvent based paint remover, and can be used on most painted or varnished surfaces.

 Test a small section to see how long the whole job is likely to take.• Apply Paint Stripper liberally by brush or roller a dry surface.• Take caution on plastics (it may harm some thermoplasts). • Don’t do too big an area at once. • Leave for 15 – 30 minutes until the paint coat dissolves or lifts. (Antifoulings, alkyd paints and varnishes react quickest, silicone and epoxy take longer.) • We recommend putting aluminium foil over the treated area to help stop active solvents evaporating. • Scrape off old paint; clean with  hot water and Pre-Clean.• Old or thick coatings may need  a repeat treatment.


Alternatives to Paint Stripper-Abrading.

Use coarse paper for coatings, being careful not to damage the substrate. Wet abrade antifoulings to avoid inhaling toxic dust particles.

Hot air guns can remove paints and varnishes, but not antifoulings as toxic fumes are released.

Don’t damage/burn the substrate!Paint scrapers work for varnishes and paints, but not antifoulings.

Key Advice:

If you’re using a hot air gun,  use at low temperature and take great care

   2. Cleaning and degreasing

Good surface preparation is the key to achieving a great finish. Part of this preparation is making sure the surface is free from dirt and contamination.

Pre Cleaning:

It is a good idea to Make sure your surface is clean before starting painting. Give the surface a good clean with detergent and/or high pressure waterblast.

Be careful when waterblasting not to get to close and damage the surface


Use Degreaser to remove surface contaminants, especially wax or silicone on new gelcoat.

Do not use it on single component conventional paint systems, as the solvents in Degreaser can damage the coating.

• Work in a well ventilated area using a clean absorbent, lint free cloth soaked in Degreaser. • Using the soaked cloth in a longitudinal motion, clean 1m2 at a time, changing the cloth surface before the next section – wear solvent resistant gloves and eye protection while doing this. • Wipe excess Degreaser from the surface using a new dry cloth.

Key Advice:

Check the surface for grease by sprinkling it with water. If pearly drops form, the surface is still greasy and needs a further treatment with Degreaser. If the water flows out evenly, no grease is left on the surface.



After cleaning the surface you’re going to paint, it must be abraded to the correct profile. This is usually called “keying” the surface. After keying, it’s essential to remove any dust before painting.


Dry abrading

Dry abrading is recommended for:• removing old paint (not antifouling) • sanding filler • initial preparation of wood, aluminium, steel, lead and GRP. Dry sanding creates a lot of dust, so you should always wear a good quality particle mask and eye protection. Dry abrasive paper is available in various grades and comes in sheets, disks or on a roll. To ensure even hand abrading, wrap the paper around a cork sanding block.

Do not dry abrade antifoulings. They must always be wet abraded to avoid inhaling toxic dust particles.

Wet Abrading

Due to the lubricating action of the water, there is minimum paper clogging and a clean surface can be quickly achieved. Wet abrasive paper is available in sheets in various grades, and should be used around a cork sanding block to ensure an evenly abraded surface.

Antifoulings must always be wet abraded to avoid inhaling toxic dust particles.

Surface to abrade Dry paper grade Wet paper grade
Gelcoat before priming for antifouling 150 180
Gelcoat before priming for topcoat 150-180 240
Bare wood 80-240  
Bare metal 60-120  
Epoxy filler 60-100  
Painted surface 150-180 180-240
Varnished surface 220 240
Old/deteriorated gelcoat 80-120 120
Hard antifouling for a racing finish   400-1200
Before final coat of varnish or topcoat 280-400 600-800


Mechanical Abrading

The most popular types are:

Belt SanderFor rapidly removing material on flat surfaces.

Random Orbital/Dual Action SandersRapidly removes material from most surfaces.

With the correct paper grades, you can use these sanders all the way from rough sanding to final sanding before topcoat application.Orbital SanderGeneral purpose sander for most preparations. Uses standard abrasive paper, making it an economic choice.

Abrasive Blasting

Blasting leaves the ideal surface for new coatings. Grit, slurry and sand are good for this. For aluminium and stainless steel, use a non-metallic abrasive, such as garnet. This is usually carried out by a professional with the right equipment, but you can hire a high pressure hose with attachments.


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