Epoxies consist of a ‘base' and a ‘curing' agent. The two components are

mixed in a certain ratio. A chemical reaction occurs between the two parts

generating heat (exotherm) and hardening the mixture into an inert, hard ‘plastic'.

Epoxies yellow, chalk (or more commonly least lose their gloss),

in direct sunlight (UV). The yellowing can be a real problem. For

pigmented epoxies select colors that are dark or contain a lot of

yellow (such as green). Even clear epoxies will yellow and cloud

up. Often epoxies are top coated with latex or urethanes that will

retain their color and attractive gloss. This is particularly true if

color coding or matching company colors is important.

Epoxies will harden in minutes or hours, but complete cure

(hardening) will generally take several days. Most epoxies

will be suitably hard within a day or so, but may require more

time to harden before the coating can be sanded.

By their nature, epoxies are hard and brittle. Additives

can be added to epoxies that make them less brittle, but

generally at the loss or reduction of other positive epoxy

properties such as chemical resistance.

Other clues of cheap epoxies include ‘induction time'

(after mixing the two components the mixture must sit

for several minutes to ‘self cook' before being applied).

The best time to recoat epoxy is within about 48 hours

after the initial coat. Because epoxies take days to reach

full cure, a second coat applied shortly after the first coat

will partially fuse to the first coat rather than forming a

simple mechanical bond.
 

End users can thicken epoxy with many things, Tiny glass

spheres, known as micro-spheres or micro-balloons are

commonly used. Besides thickening, their crushable nature

makes sanding the hardened epoxy easier. On the downside,

they work like tiny ball bearings, resulting is sagging and slumping.

Another thickener is fumed silica (a common brand name is Cabosil (tm))

which looks like fake snow. About 2 parts fumed silica with one part epoxy

will produce a mixture similar in texture and thickness to petroleum jelly.

Micro-spheres and fumed silica can be combined together.

Fisheyes are areas on a painted surface where the coating literally

pulls away for the substrate leaving a coatingless void or fisheye.

Often fisheyes are caused by surface contaminants such as a bit of

silicon, wax, or oil. I have also seen them on clean plywood where

epoxies paints have been used as sealers and the problem might

be due to uneven saturation (soaking-in) of the epoxy into the wood.

Surface tension plays a big part in fisheyeing. There are some

additives that can be mixed into the epoxy that will reduce

surface tension. Likewise, on wood, applying several coats of

solvent thinned epoxy, instead of one coat of unthinned epoxy,

seems to work well. Applying a thick coat of epoxy over a

contaminated fisheye surface will bury the fisheye but expect

the coating to peel away in the future. As a rule of thumb,

always suspect some sort of surface contamination as the

primary cause of fisheyeing.

Adding a bit of solvent to a solvent based or solvent-free epoxy

is something that most manufacturers would not officially approve

of and something that might not work with all epoxies. However,

it can be done (unofficially) with the epoxies I deal with. Adding

solvent to these epoxies will: 1) thin them out; 2) increase pot life;

3) allows them to flow off the brush/roller a bit more smoothly; and

 4) perhaps allows them to ‘soak-in', penetrate, or may be soften,

the substrate just a little bit. Not change is visible in the epoxy

unless 12% or greater solvent is added. With that amount of solvent,

the epoxies no longer cure with a glossy finish.

It is best to use epoxies with a mix ratio close to 1 to 1 as opposed to

something 4-1, 5-1, etc. because errors in the mix ratios can be more

pronounced with the latter. That said, no matter what the mix ratio is,

some epoxies are more forgiving of mix ratio errors than others. One

‘trick' of epoxy vendors with odd or very sensitive mix ratios is to sell

calibrated pumps that disperse the epoxy components in exact amounts.