Most craft of whatever size or construction material have required filling and fairing compounds at some time either in their life or in their development and design. Production grp hulls which emerge from their moulds with a smooth gel coat finish owe their fairness and quality of finish to the skill of the builders original shape or ‘plug’ from which the mould was formed.

The same high standard of finish can also be seen in ‘one-off’ composite racing sailboats. These craft mostly use epoxy rather than polyester resin in their construction and are built over a male plug. All one-off racing sailboats use lightweight epoxy filler mixers and specialised high performance paint schemes to obtain the necessary high standard of finish.

Recently we have seen an increase in the number of craft, both mono and multihulls, which use wood as the principal core material in their construction, particularly cedar strip and end-grain balsa. These hulls rely on extensive filling and fairing to give a surface finish which can match that of moulded grp hulls.

Steel boat builders also use epoxy filler to fair their hulls once the shaped steel plates have been welded together. Filler will take out any slight curvature imperfections and hide the welding lines before a final paint system is applied.

Filling and fairing is a very labour intensive operation but an essential one necessary to ensure that the hull offers the least resistance to movement through the water. Nowhere is efficient filling and fairing more important than on underwater appendages - the keel and rudder, where not only section shape can be controlled by application of filler, but surface finish, fairness and smoothness can be perfected for noticeable performance gains.

This guide covers the range of products which are available used in filling and fairing procedures and outlines the standard procedures which are necessary in order to achieve the best quality finish on wood, composite, ferrocement and metal surfaces.


Preparation Guide for Filling on Different Substrates


Wood is an absorbent material and therefore the best possible adhesion is obtained by first pre-coating with an unfilled solvent free epoxy resin and hardener mix. The filler mix can then be applied either after first leaving the surface to cure and then sanding to obtain a good key, or whilst the initial unfilled epoxy coating is still wet or tacky. Deep indentations or screw holes may be filled without pre-coating.

Polyester GRP

  • Fibrous Laminate Surface without Gel Coat

  • Pre-coat with clear, unfilled solvent-free epoxy resin mix first and treat as for bare wood. Polyester grp should be well cured otherwise the epoxy cure will be inhibited and there will be poor adhesion to the surface.
  • Polyester Gelcoat Surface

  • Wet sand with 150 - 180 grit paper then solvent wipe with a fast evaporating epoxy solvent (e.g. SP Solvent A) to obtain a good key. The polyester should be well-cured.

Polyester Filler or Epoxy Filler

A previously filled surface should be well abraded with 40 - 80 grit production paper and wiped with fast evaporating solvent (e.g. SP Solvent A) before commencing any further filling. The polyester should be well-cured if epoxy filler is to be used.

Epoxy FRP

This may or may not have used nylon peel ply incorporated into the last laminate layer. If it had, then tear away the peel ply and simply apply the filler. If no peel ply was used then sand the surface thoroughly with 80 grit production paper and clean with SP Solvent A (Fast Epoxy Solvent) or other fast evaporating solvent such as clean acetone.


  • Sand-blasted For the best possible adhesion, first wipe with Solvent A then pre-coat with a thin unfilled solvent-free epoxy resin mix. Then apply filler mix whilst this coating is still tacky.
  • Primed Metal primers should ideally be epoxy based and be well sanded, followed by a wipe with fast evaporating solvent to give the epoxy filler a good key.


First check that no metal reinforcement is standing level or proud of the surface. Clean off laitance and preferably wash with dilute hydrochloric acid. Brush off and dry, then treat surface as for bare wood.

Filler Guide

Apart from premade available filler there is the option of making your own using various filler powders available, this enables the user to mix for a particular purpose. This is ideal if you are using one resin for a project and need a variety of requirements from it, laminating, sheathing, filling and bonding.

Hollow Sphere

Hollow spheres can be generally described as ‘microspheres’. They serve to increase the volume and reduce the density of any filled resin system to make adhesive mixes and filling & fairing mixes. Since microspheres serve to ‘displace’ resin and do not absorb resin into their internal cavities they are useful for creating low density filler mixes, core bonding adhesives and syntactic foams for cosmetic and structural applications. Within this category there are two different types of microspheres:

Gurit Microballoons

gurit microballoonsGurit Microballoons are hollow phenolic resin spheres which have a distinctive reddish/brown colouration thus making them particularly useful for cosmetic fillet joints and fillers in wood construction, as well as structural adhesives for low demanding applications on softer timbers such as cedar. Although not as waterproof as glass bubbles, microballoons are often preferred for their excellent sanding characteristics. Microballoons are not normally used with polyester or vinylester resins because they can be subject to styrene attack which may cause the spheres to collapse.
When storing microballoons, it is particularly important to exclude air as they readily absorb atmospheric moisture which will affect the performance of the filled mix.

Composition: Phenolic Resin 
Appearance: Red/Brown Powder, occasionally sticky
Particle Size: 50 microns
Bulk Density: 100g/litre

Gurit Glass Bubbles

Gurit Glass bubbles are hollow glass spheres with a more variable particle size than microballoons. Being composed chemically of glass, they are physically harder than microballoons and filled resin mixes are noticeably more difficult to sand. However, glass bubbles produce a more waterproof filler mix and are often used on below- waterline applications on boats. Being significantly less expensive than microballoons they are often preferred if ultimate sanding performance and colour are not of prime importance. They can be mixed with microballoons in any proportion for colour purposes.

Composition: ‘C’ Glass
Appearance: White powder, free flowing
Particle Size: 40 - 80 microns
Bulk Density: 100-150g/litre

Gurit Microfibres

micro fibres guritGurit Microfibres are very fine wood-cellulose fibres commonly used to create structural adhesives for bonding both wood and grp. Because any low viscosity resin system is readily absorbed into a wood surface, an unfilled adhesive may have a tendency to give a ‘dry joint’. Because of their absorbent properties, microfibres can retain a significant quantity of adhesive within a joint and limit resin absorption into the surrounding wood fibres, thus ensuring an adequate resin supply for adhesion. Where the strongest bond is required, e.g. scarf joints on any type of timber, microfibres should always be used on wood joints, in preference to hollow sphere types of filler.
For bonding parallel to the grain with lower density, lower strength timbers, such as cedar or obeche, a microballoon mix is adequate.

Composition: Milled bleached cellulose wood pulp
Appearance: White 'fluffy' fibrous consistency
Particle Size: 200 - 300 microns
Bulk Density: 100g/litre

Gurit Ampro Colloidal Silica

gurit ampro colloidal silicaGurit Colloidal Silica (or Cab-o-sil) is an agent which is used to control the thixotropy or ‘sag’ characteristics of a resin system. By adding colloidal silica in varying amounts to a resin mix containing the other filler types mentioned, the handling characteristics can be controlled. Relatively small quantities added to a resin mix containing glass bubbles or microballoons will give non-sag properties and impart easier handling. Colloidal silica is also added with microfibres to produce a mix suitable either as a high strength non-sagging structural adhesive (particularly for non-absorbent materials such as grp), or as a ‘high density’ filler. The inclusion of colloidal silica has the effect of increasing the hardness of the resulting mix which will create more difficulty when sanding. For this reason colloidal silica is usually added in the minimum quantities to any mix for which sanding is anticipated. In some applications this feature can be used to advantage, to create a hard wearing edge or surface.

Composition: Silicon dioxide
Appearance: White powder, easily airborne
Particle size: 0.012 microns
Bulk Density: 50g/litre

Select the additive(s)....Micro BalloonsGlass BubblesMicro FibresColloidal Silica
Filling & Fairing MixesEasy to sand, filing and fileting for cosmetic wood applications
Water-proof or lowest cost filler for general applications
Hard surface finish filler for hard wearing edges and surfaces
Bonding MixesGeneral bonding of softwood, brown in colour
General bonding of softwood, white in colour
Structural bonding of wood or composite, opaque in colour