Protective Coatings for Wood

Topics Covered

Background

Coating Wood - A Two-Stage Process

Stage 1 - Primer Schemes

Solvent-Free Epoxy Coatings

Solvent-Based Epoxy Systems

Stage 2 - Finishing Schemes for Clear or Painted Finishes

The Cost Effectiveness of Coating with Epoxy

Background

This guide has been written specifically with wooden boats in mind. However, the rationale for the various coating schemes and the techniques employed can be applied to any wooden structure.

Being an absorbent fibrous material, wood has traditionally meant constant high maintenance, particularly for the marine user, where the cycle of moisture uptake-and-loss brings about seasonal dimensional changes.

With traditional designs, the oil-based coatings employed were able to be flexible and withstand a certain amount of movement without cracking. However, deterioration inevitably occurred, particularly on seams and joints, which often lead to more serious structural deterioration. Intensive annual maintenance was the only answer. Today, new wood building methods have been widely adopted to create structures far lighter, yet stronger and more rigid, than their predecessors. These new methods, which can be traced back to the advent of glued wooden aircraft structures, take advantage of modern adhesive technology and, more specifically in recent years, to the development of marine epoxy systems for gluing and coating.

Building in plywood, cold-moulding with thin veneers, or strip planking with longitudinal rectangular section timbers, are the basic methods which can be used to build today’s ‘minimum maintenance’ wooden components and structures.

Whereas the above methods of build lend themselves well to epoxy glues and coatings there are obviously some older methods which do not. In general, epoxies should not be used to completely coat boats where movement between adjacent ‘planks’ may be expected. This includes traditional clinker and carvel construction. The control of moisture is central to the success of these techniques if they are to be considered as long term viable boat building methods. Excessive moisture degrades wood both structurally and biologically. Wood is strongest and stiffest when it is dry (under 15% moisture content is the practical limit). With wood/epoxy building methods, wooden boats can be built which are stiff, light and durable and which, with the correct coating scheme, can remain almost maintenance-free for years.

Coating Wood - A Two-Stage Process

Stage 1 - Primer Schemes

Most people will be familiar with the concept of primer, undercoat and topcoat in painting. The primer serves to provide an improved bonding link between the paint and the surface to be covered (substrate). It also helps to fill in any imperfections in the substrate. This provides a smoother surface for the subsequent paint coats, which in turn allows faster and better quality finishing.

The undercoat and topcoat then work together to provide a barrier to anything which may attack and degrade the surface to be coated. Protection is therefore afforded against moisture, pollution and biological entities such as rot, mould, boring insects, etc. The barrier is provided by a combination of the chemistry of the paint (acrylic, polyurethane, oil based, etc.) and the thickness of the coating. The undercoat provides the build up of thickness and the final topcoat provides the 'cosmetic' finish required - colour, gloss, texture, etc.

Epoxy provides a coating with the best possible adhesive qualities and some of the best possible protective qualities, particularly against water and aggressive chemicals. For these reasons it is the preferred protective coating for the oil industry, to protect steel from attack by seawater (offshore rigs), aggressive chemicals (oil refinery tanks and pipes), and physical damage. However, epoxy also bonds extremely well to both concrete and wood surfaces. Because of the excellent adhesive and protective nature of epoxy, it is ideally suited to the primer stage of a coating scheme. In this role, the number of layers of 'cosmetic' topcoat can be reduced and it is possible to eliminate the undercoat altogether, giving just a two stage process.

Solvent-Free Epoxy Coatings

Solvent-free epoxy coatings are noticeably more demanding than conventional paints in terms of temperature requirements (though not particularly as sensitive as adhesives) and their handling procedures require more attention. For instance, accurate metering and mixing of resin and hardener components is essential and the dissipation of heat generated by the reaction demands careful handling in order to gain adequate pot-life and working time.

The specific advantages that a solvent-free epoxy system provide are judged by many to make them irreplaceable for certain applications. For example, because of the greater coating thickness possible, fewer coats are required. This reduces the labour content necessary to achieve a given dry thickness. Furthermore, fibre reinforcement can easily be incorporated into the coating to provide additional strength - this is not possible with solvent-based coatings.

In addition, solvent-fee epoxies exhibit no noticeable shrinkage on curing and can contribute significantly to the stiffness of thin plywood panels (2-6mm thick). The term ‘wood epoxy saturation’ is commonly used to describe their use though it is strictly a misnomer as only the surface fibres become ‘saturated’, the extent of saturation rarely extending more than 0.5-1.0mm. Typical dry film build is 100 microns.

Solvent-Based Epoxy Systems

The solvent based epoxy systems behave in a manner more familiar to the average user. On application these products are more like a two-part paint or varnish, benefiting from longer pot and working life and greater tolerance to poor conditions such as low temperature (less than 15°C) and high humidity (more than 75% RH).

However, at least three times as many coats are required for equivalent dry film thickness (typically 25 microns per coat). This is the thickness after the solvents have evaporated since such coatings characteristically shrink at least 50% on curing. Solvent-loss, although detrimental to film build, does permit a flatter finish per coat. The higher number of coats which are required and the time which must elapse for the release of solvents before recoating can result in longer finishing times where high build, i.e. complete filling of wood grain, is required. The need for the evaporation of solvents means that solvent-based epoxies cannot be used as glues.

Table 1. Choosing the most appropriate system

Factor

Solvent Based

Solvent Free

Skill Required

Low

Moderately High

Warm Conditions

Not So Important

Very Important

Dry Coating Thickness

Low To Moderate

Very High

Added Stiffness

Low

Very High

Shrinkage

Moderate

Very Low

Special Conditions

None

Localised Heating May Be Required

Odour Level

Very High

Very Low

Durability

High

Very High

Filling Ability

Poor

Very Good*

Use on Marine Craft Parts

Mainly Decks and Interiors

Hulls, Decks and Interiors

*Solvent-free epoxy can repair surface splits in plywood and be used as a general construction adhesive

Stage 2 - Finishing Schemes for Clear or Painted Finishes

After the application of the epoxy coating ‘primer’ either clear or paint finish products will be required. For a clear finish, two-pack polyurethane varnishes will give a very hard wearing, protective finish. They will both reduce the deterioration of the surface of the underlying epoxy (caused by ultra violet in sunlight) and give a high gloss surface finish.

When a painted finish is required a good undercoat is required to both to obliterate surface blemishes and to give a good colour base. Overcoating with a two-pack polyurethane paint is the best scheme for long term durability and protection, but single component polyurethane or oil-based paint can equally be used.

The Cost Effectiveness of Coating with Epoxy

The highest quality finishes are usually those with a sufficiently high build up of coating to give the wood protection against both moisture ingress and against abrasion and knocks. Obviously solvent-free epoxies are ideal for this application and the necessary thickness can be achieved in only three or four coats. This protective layer is then followed by either varnish or paint in a relatively thin surface coating. For varnished finishes, material cost savings of over 50% can be made using epoxy and two-pack polyurethane varnish, compared with using only a two-pack varnish to achieve the same build. For painted finishes the savings are less (only about 10%) since there is a higher proportion of solvent-based product included in the scheme.

Source: SP Systems. This article was derived from ‘Wood Boat Protective Coat.pdf’ which can be downloaded from the SP Systems website.

For more information on this source please visit SP Systems

 

Date Added: Dec 13, 2001 | Updated: Jun 11, 2013
Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit