SECTION VII A - COATING WITH EPOXY RESIN
Wood is often coated with epoxy to dimensionally stabilize it and provide a barrier which helps to prevent the passage of moisture. Silvertip Laminating Epoxy has a certain amount of flexibility and tough resilience built into the formulation. Because of this, a plywood panel could be coated on the bench, then bent into place without danger of the epoxy cracking. When working flat you're not fighting gravity and the coated panel is easily sanded on the bench using a disc sander and foam pad. The sanded panels are then installed and are ready for painting. Coating a 4'x8' sheet of fir plywood will illustrate this method:
Mix the SilverTip resin and hardener in the correct ratio, referring to Appendix A to estimate the amount you'll need. Pour this mix on the plywood in a stream of "S" curves starting at one end and finishing at the other, making four or five curves along the eight foot length. Spread the epoxy back and forth with a squeegee into the dry areas, trying to get as even a coating as possible without being too fussy. Use a dry foam roller to even out the coating. When this first coat is cured to at least a soft set tack free stage it can be recoated. Subsequent coatings applied at anytime between this soft set stage and 72 hours do not need to be sanded and will chemically bond.
Subsequent coatings may still bond well after 72 hours without sanding but the proposition gets riskier. An amine cured epoxy surface is quite alkaline and can react with any acidic material such as moist carbon dioxide or silicates. Further epoxy coats may not bond well to some of these reaction products. Sanding, in addition to providing some "tooth" for mechanical bonding, also cleans since it exposes new, uncontaminated surface. If in doubt, sand enough to kill most of the gloss.
Working on non-horizontal surfaces is similar except that the mixed resin is poured into a roller pan and applied with a foam roller. To control runs and sags use several thin coats rather than a few thick coats. As with coating the flat panel, just wait until one coat has reached the soft set stage before applying the next.
In boat building use at least two coats for interior wood surfaces and three in areas that may be constantly wet, such as bilges. Darkroom sinks should have at least three coats on the wet side and two on the back. White epoxy paste pigment is a nice addition to an epoxy coating where appropriate. Unlike paint it ill not flake off.
Several tricks can be used to improve the appearance of the finished film. Bubbles that persist in the coating can be broken with a foam brush by lightly dragging it across the surface. Quickly and lightly fanning the uncured surface with a propane torch will accomplish this with greater speed. Avoid over heating an area as this could cause the epoxy film to pull away from the surface creating craters. Overheating will also cause the expansion of any air in the pores of the wood and may result in an epoxy coating full of bubbles.
Sometimes a coating will try to crater. This is most common with recoated surfaces that have been sanded, but may happen on other surfaces as well. While the cause of cratering is quite complex, the solution is pretty simple. Immediately after coating a surface look at it from an angle, sweeping your eyes over the whole surface. Craters will usually form within ten minutes after first applying the coating. Take the heel of the foam roller and really grind it in the area that has cratered. This wets out the dry spots in the crater center. Then, re-roll the area treated to even out the coating.
After 24 to 48 hours (depending upon temperature and hardener used) the coating will be cured enough to sand. When using general-purpose resin first wipe the surface with a damp sponge to remove any water-soluble amine carbonate surface film prior to sanding. It is not necessary to wipe SilverTip Laminating and Coating resin prior to sanding. At this stage of cure the epoxy coating can usually be sanded with a disc sander or random orbital sander. Machine sanding can generate quite a bit of heat, especially when the sandpaper gets dull or clogged, causing gumming of the sanding dust. Keep the sander moving and apply only light pressure. This keeps the heat down. If clogging still happens you'll have to either hand sand, scrape as described below, wet sand or allow another day for the cure to proceed.
Scraping is an alternative to sanding that actually produces a better finish. This shaves off a thin film of epoxy leaving a surface that looks like it was sanded with 600 grit paper. Small parts can be scraped using a single edged razor held vertically. Several companies make wood scrapers for working on larger surfaces. Keep them sharp and be careful not to cut yourself. Any residual sanding dust should be removed by blowing or brushing it off prior to recoating. The final bit may be removed with a damp rag. Don't use acetone, other solvents or tack rags. They may leave an unbondable surface coating on the sanded epoxy surface.
Try to work at a constant or falling temperature when coating new wood. When the temperature is rising, air trapped beneath the uncured epoxy may expand and cause small bubbles to form in the coating. Avoid working in direct sunlight on new wood for this reason. If you must work in sunlight, coat the wood as the sun is going down. The wood will be cooling and air bubbles should not form. Cover any outdoor work to help prevent dew from forming on the uncured epoxy surface.
Some very porous woods are quite persistent at forming air bubbles. A trick we have used is to heat the whole surface to a temperature at least 40°F higher than room temperature. Use a hot air gun or place the wood in the sunlight for a while. Stop heating and immediately coat the surface.The epoxy will thin on the warm surface and at the same time start to cool it. The air in small pores will begin to contract pulling the thinned epoxy in to them. Any air that does rise will be going through thinned epoxy and have an easier time of it. In lieu of this you may be able to apply a thin epoxy coating, allow it to soak in and then squeegee and discard any remaining on the surface. Once cured the coated wood now acts as a non-porous surface and rising air bubbles should pose no further problems.
Clear Coat epoxy can also be used for coating wood. Like SilverTip Laminating Epoxy it leaves no amine blush on the surface. It is a much thinner material and, while an argument could be made that this is good for the first coat, it takes over twice as many coats to achieve the film thickness and hence moisture barrier protection of SilverTip or our general purpose epoxy.
SilverTip Laminating Epoxy is an excellent base for varnish. The application of multiple coats of varnish and sanding between coats can be eliminated with two coats of SilverTip with NO sanding between coats. The final epoxy coat is sanded to provide a base, and then one or two coats of varnish are applied. The result appears to have the depth of ten or more coats of varnish and is much more durable. Revarnishing is much easier because the old varnish is just removed down to the epoxy coating. SilverTip Laminating Epoxy may be thinned with up to ten percent lacquer thinner to improve brushing. The use of solvents will retard the cure time somewhat so don't use any more than needed. Add just enough thinner to allow the epoxy to brush easily. Never add solvents to epoxy for gluing or fiberglassing.
Clear Coat epoxy is also used as a base for varnish but has several differences from SilverTip Laminating Epoxy in this application. First, it is thinner and can be easily brushed without adding solvents. Second, it is much slower affording longer working time but at the expense of a longer cure time. Third, it soaks into wood much better. Like SilverTip Laminating Epoxy it does not need to be sanded if recoated within two or three days. Unlike SilverTip Laminating Epoxy it takes over twice as many coats to achieve equal thickness.
Clear Coat epoxy may water spot if water stands on it even though it has been cured for a long time. This is a phenomenon unique to the raw materials used in the Clear Coat hardener. Sanded Clear Coat epoxy will not water spot as the offending surface layer has been removed. SilverTip Laminating Epoxy develops water spot resistance within 24 hours of application.
Epoxy coatings should be sanded before varnishing or painting. These materials stick to the epoxy by mechanical means and must have some "tooth" in order to bond well. Never apply solvent based coatings to partially cured epoxy. Read the Painting Section before painting or varnishing an epoxy coating.
Materials Required for Coating:
- SilverTip Laminating Epoxy (Resin and Hardener)
- Foam roller covers/frames
- Measuring device
- Plastic squeegees
- Protective gloves
- Brushes, foam and bristle
- Tags: The Epoxy Book
- STR Staff