Final Fairing & Finishing - WEST SYSTEM Epoxy

Finishing 18
Adhesion
Paint adhesion relies on the mechanical grip of the paint keying into the sanding
scratches in the epoxy’s surface. Big scratches provide the best adhesion, but they may
show through the finish. As a general rule, use no coarser than 80-grit with a mechanical
sander and no coarser than 100-grit when hand sanding. The final grit you use depends
on the type of paint and how thick a film will be applied. Try to use the coarsest
paper that will not show scratches through the paint.
Surface contamination is a common cause of adhesion problems. After an epoxy barrier
coat is properly sanded, protect the surface from all potential sources of contamination
such as engine and heater exhaust, hand and finger prints, and rags cleaned or
dried with fabric softeners. To reduce the possibility of contamination, begin the final
coating application as soon as possible after the barrier coat preparation.
Compatibility
Most types of coatings are compatible with epoxy. Thoroughly cured epoxy is an almost
completely inert hard plastic. This means most paint solvents will not soften,
swell or react with it. However, it is still a good idea to build a test panel to assure coating
compatibility and work out application procedures before final application on a
project. The test should duplicate the barrier coat finish, finish coating materials, application
procedure and shop conditions. A lot of variables can affect the success of the
final coating. Working out the bugs beforehand can save money, time and effort.
Note: Some types of one-part polyurethane coatings may not cure properly over epoxy.
Incompatibility may be a result of a specific polyurethane catalyst reacting with or being
affected by amine blush on the surface of the epoxy coating. Cure mechanisms vary from
brand to brand. If a specific brand is incompatible, try a different brand or a different
type of coating or use 207 Hardener. 207 Hardener is compatible with most coatings.
Hardness
The hardness of a coating is another factor to consider in selecting a finish. Manufactures
refer to coating hardness in terms of a “pencil” hardness scale: 6B (the softest), to
HB (the middle), to 6H (the hardest). Harder coatings are more resistant to wear and
last longer than soft finishes. It is easier to remove flaws or scratches in harder finishes
by rubbing or buffing. (Some coatings may harden enough to buff only after weeks or
months of curing.)
The biggest advantage of a hard glossy coating is that it is easier to keep clean. Softer or
more flexible coatings hold dirt easier and require more abrasion to clean. The more
they are abraded, the more dirt they hold, and the more abrasion is required to clean
them the next time. The trade off is that too hard a finish can chip or crack if it’s applied
to a flexible surface.
Application considerations
The method of application is another important factor in the selection of a coating.
Spray application is preferred if achieving the smoothest possible finish is the primary
goal, but in some situations rolling, brushing or a combination of rolling and brushing
may be your preferred or only choice. Spraying depends on access to proper equipment
and an environment where overspray and solvent fumes can be controlled. Safety
and health considerations are a major concern when spraying, especially with coatings
like linear polyurethanes which require the applicator to wear special breathing
equipment.
Roller/brush application is a good alternative to spraying. The same technique described
for applying epoxy in Epoxy Barrier Coating (page 13) can be used to achieve a
reasonably smooth finish with most coatings. There are a number of advantages to this
method. The cost of transporting large projects to the nearest suitable location for
spraying can be prohibitive. Most coatings are much less hazardous when they are not