Boat Undercoat Paints

Boat undercoat paints are a misunderstood and often ignored part of painting boats. The undercoating stage of boat painting is a vital step to ensure a professional finish which will last for many years.

Contact us here for boat painting and other boat repairs, or call 07886 388 689

Boat Undercoat Paint – what is it?

When painting a boat, it is important to pick a paint manufacturer and stick to their paint system. There is the risk that two different manufacturers systems are not compatible. If this is the case, you will find peeling topcoats, sagging primers and a generally poor finish. If you have spent days or weeks preparing the boat for painting, it would be a shame to have wasted your time. We generally recommend Teamac or Craftmaster boat paints.

When choosing your paints and picking colours, you will notice a category called ‘undercoats’. We find many people we talk to understand what primer is for, and topcoats are more apparent, but are uncertain about boat undercoat paint.

Applying Boat Undercoat Paints - The Floating Boatyard

The role of primers

To understand the role of undercoats, we should look at a standard paint system overall:

Primers are applied to untreated surfaces, such as bare wood or steel. Their role is to stick to the material being painted and to provide a surface for the undercoat to hold to. Primers generally do not cover many flaws such as pitting and scratches in the steel. Primer is porous: This gives good adhesion for the following boat undercoat paint. This does also mean that primer does not protect the steel or wood from water.  We cover primer coats in more detail here.

Boat undercoat paints – the undercoat itself

After priming, we apply undercoats to all of our boat painting projects. Boat undercoat paint is typically high build. This means that it deposits a thicker layer, which helps cover up minor pitting and scratches that are too small to be filled by other methods. Once the boat undercoat paint has been applied, we sand using very fine grade paper to get a flat finish.

The undercoat also provides a better surface for the topcoat to adhere to, it is less coarse than the primer. Topcoat which has been painted straight onto primer will generally fail faster than a project where undercoat has been used.

Boat undercoat paints are also used to correct the colour before the topcoat. Primers are generally dark grey (zinc oxide primer) or red-brown (iron oxide primer). Both of these colours will show through gloss topcoats, affecting the look of the final finish. Brighter and deeper colours of topcoat are counter intuitively more transparent than they would appear. In our experience, deep blues and bright reds especially suffer from this. It is important that you get the right colour of undercoat before applying topcoat to get the colour to look as expected. Undercoats come in a range of colours and your paint manufacturer will recommend the correct colour to use.

Boat undercoat paints – taping up

Boat undercoat paints are applied after the primer coat and before the topcoat. Once the boat has been primed and filled, you can begin undercoating. If the topcoat colours you have chosen require more than one colour of undercoat, for example for panels or coach stripes , you will need to mask these areas off at this stage.

Generally, you want to paint the lightest colours first, in case of any accidents or bleed under the masking tape. Use the best quality masking tape you can find. Use ultra low-tack tape, if you are painting outdoors then tape with UV-resistance is necessary as sunlight can affect its glue. Take off the tape very carefully as soon as the paint is touch dry. The longer the tape is sat on your freshly painted and undercoated surfaces, the more likely it is to pull off the paint underneath it.

Sanding the undercoat

We apply the required number of coats and sand carefully. At this stage, it is important to us a very fine sandpaper possible. The purpose of sanding at this point is to remove imperfections and foreign bodies, as well as flatten the brush marks in the undercoat. If the sandpaper is too coarse, you will remove the boat undercoat paint entirely. In extreme cases and on uneven surfaces it is possible for coarse paper to remove all paint applied, meaning that the boat will have to be primed again and then undercoated.

It is vital to use a professional random orbit sander with dust extraction to do this. Any scuffs or sanding marks left will be very visible in the topcoat. Pay particular attention to weld lines, often small drips appear at these. When left unsanded, these encourage larger drips to form in the topcoats where remedial work is much more difficult.

Ultimately, undercoats are no less important than primers or topcoats when painting narrowboats. Our team of professional boat painters approach every step of painting with the same care and attention.

Always get permission form you local waterways authority before working on your boat while it is in the water.

Contact us here for boat painting and other boat repairs, or call 07886 388 689



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