The object of barrier coating is to build up an epoxy coating that provides an effective moisture barrier and a smooth base for final finishing.
Apply a minimum of two coats of WEST SYSTEM epoxy for an effective moisture barrier - three coats if sanding is to be done (recommended for most wooden boats). Moisture protection will increase with additional coats, up to six coats. Six coats, with 422 Barrier Coat Additive in the final five coats, provides maximum moisture protection (recommended for polyester fibreglass boats with gelcoat blisters). Additives or pigments should not be added to the first coat. Mixing thinners with WEST SYSTEM epoxy is not recommended.
Disposable, thin urethane foam rollers, such as WEST SYSTEM 800 Roller Covers, allow you greater control over film thickness, are less likely to cause the epoxy to exotherm and leave less stipple than thicker roller covers. Cut the covers into narrower widths to reach difficult areas or for long narrow surfaces like stringers. A paint brush can be used for smaller areas, if the bristles are stiff enough to spread the epoxy to an even film. If necessary cut the bristles shorter. Foam brushes are generally too flexible.
Complete all fairing and cloth application before beginning the final coating. Allow the temperature of porous surfaces to stabilise before coating. Otherwise, as the material warms up, air within the porous material may expand and pass from the material (out-gassing) through the coating and leave bubbles in the cured coating.
1. Prepare the surface for bonding.
2. Mix only as much resin/hardener as you can apply during the open time of the mixture. Pour the mixture into a roller pan as soon as it is mixed thoroughly.
3. Load the roller with a moderate amount of the epoxy mixture. Roll the excess out on the ramp part of the roller pan to get a uniform coating on the roller.
4. Roll lightly and randomly over an area approximately 600 x 600mm to transfer the epoxy evenly over the area.
5. As the roller dries out, increase pressure enough to spread the epoxy into a thin even film. Increase the coverage area if necessary to spread the film more thinly and evenly. The thinner the film, the easier it is to keep it even and avoid runs or sags in each coat.
6. Finish the area with long, light, even strokes to reduce roller marks. Overlap the previously coated area to blend both areas together.
7. Coat as many of these small working areas as you can with each batch. If a batch begins to thicken before it can be applied, discard it and mix a fresh, smaller batch.
8. Tip off the coating by dragging a foam roller brush lightly over the fresh epoxy in long, even, overlapping strokes after each batch is applied. Use enough pressure to smooth the stipple, but not enough to remove any of the coating . Alternate the direction in which each coat is tipped off, 1st coat vertical, 2nd coat horizontal, 3rd coat vertical, etc. An 800 Roller Cover can be cut into segments to make a tipping brush.
Apply second and subsequent coats of epoxy following the same procedures. Recoat when the previous coat is still tacky (about as tacky as masking tape). To avoid sanding between coats, apply all of the coats when previous coats have cured to this tacky stage. After the final coat has cured overnight, wash and sand it to prepare for the final finish.