Concrete Cabochons Part Three: Epoxy Resin Topcoats
Epoxy resin gives a polished, high-gloss, wet-look finish which makes details cleare and colors deeper
and more brilliant.
Think riverstones in water as opposed to dry on the shore.
The resin can also help physically strenghten the piece or be used as an adhesive to bind pieces together.
There's a huge range of epoxy resins on the market, not all of which are suitable for our purpose.
What we want is an epoxy which will set clear, without a yellow cast. I also prefer a working time of
at least 20 minutes so I don't feel like I'm racing the clock.
You can purchase resin at many large home improvement stores (like Home Depot),
but you DO NOT want the five-minute epoxies they sell amongst the glues - these are designed to set quickly
(too quickly for our purposes) and tend to have a milky or yellowed cast.
Instead, you want the 'barstool' epoxy - likely sold near the wood stains and finishes.
Or you can check out the larger craft or jewelry supplies such as
Rio Grande, with their
resins, or Envirotex Lite,
which I believe is available through Michaels.
Even Amazon offers a
Easy Cast Clear Casting Epoxy Enamel Resin
which looks like it would work.
- Two Part Epoxy Resin
- Waxed Paper
- Disposable plastic cups
- Palette knife
- Plastic spoon(s)
- Rubber gloves
- Wet Wipes
- Heavy weight rubber bands
- Painters' tape
- Mold Release
Two resins poured over a black background. Bottom sample was a 5 minute epoxy - note the milkiness and lack of clarity.
Clear plastic drinking cups for measuring out resin and activator
Spreading the resin with a palett knife
Using an embossing gun to pop bubbles in the resin
Once mixed, epoxy resin is a thick, viscous fluid about the consistency of honey.
Self-leveling, it will form a layer about a sixteenth of an inch thick which will create a lovely,
thick topcoat for most situations. However, sometimes you might want a deeper layer, in which case you will
need to create a barrier or dam to contain the epoxy until it cures.
So, start by evaluating your cabochons. If the tops are relatively flat,
a simple topcoat will work beautifully. If they're more dimensional,
especially with protrusions rising above the main surface, you may want encircle your
cabochon with a heavy weight rubberband or painters' tape treated with mold release.
Your other alternative would be to build up the surface in multiple layers.
But a safety note with deeper pours: most resins warn that 1/4 to 1/2" is
about the deepest you should make a single pour. This is because epoxy heats up while
curing (it's a thermal reaction) and the deeper the pour, the more heat is generated. Too
deep and the heat can create a fire hazard. Not what we want!
Lay all of your cabochons on a flat, level work surface covered with waxed paper.
(Epoxy is, amongst other things, a glue and sticks to most surfaces: waxed paper is one of the
very few that it doesn't.) Now we're ready to mix our epoxy.
Mixing the Epoxy
Most epoxy resins come in two parts - the resin and a hardener - which must be mixed in equal
proportions to start the thermal reaction. It is important to mix the proportions exactly.
If you get the ratios off, the epoxy may never cure
completely. The epoxy remains slightly soft
and sticky, even days and months later, rather than hardening to a glass-like surface.
This is not what you want - trust me, I know. You may be able to salvage the situation by topcoating
with a more carefully mixed, layer, but prevention is a far better solution.
Safety recommendations: work with the epoxy in a well ventalated area, preferrably not your kitchen and
consider wearing gloves. The resin can have a somewhat offensive odor until it's set (not terrible, but noticeable)
and has been known to cause skin irriation.
If you only need a small amount of epoxy, to coat 2-3 cabochons, start by tracing two circles of the same size
(quarter or half-dollar works well) onto a sheet of paper, with an inch or so of space between the two circles.
Slip the paper underneath a sheet of
waxed paper. Pour the resin onto one
circle, being careful to stay within the border and the hardener onto the other. Give both a moment to settle out and
expand to make sure the two circles are indeed equal. Adjust as needed.
When you've measured equal proportions, mix the two together quite thoroughly with a plastic palette knife.
Bubbles will form as you mix, just
try to keep them to a minimum.
If you plan to coat more than a couple of pieces, you will want to measure the two parts into disposable plastic cups.
Mark each cup beforehand at the same level.
The resin goes a surprisingly long way, so be careful not to measure more than you actually need.
I find that 1/4" in either cup is usually enough to topcoat at least a dozen cabochons, often more.
As you measure, pour just a tiny bit more hardener than resin. I can never manage to scrape all of the
hardener out of it's cup and into the cup with the resin, so this helps keep the proportions equal.
Pour the hardener into the resin. The resultant mix will have milky streaks.
Stir thoroughly until the mixture is completely clear, making sure to scrape the sides & bottom.
Try to keep bubbles to
a minimum, but know that it's impossible as far as I can tell to prevent them completely.
Once the mixture looks clear you are ready to pour.
Pour or use a palette knife or plastic spoon to ladle a dollop of epoxy into the center of your first design.
If you're pouring, stop when the resin is about 1/4" from the edge of your cabochon. The resin
will continue to spread once you stop pouring. Use your palette knife to help spread the resin, especially
if your cabochon is an irregular shape, rather like frosting a cake.
Once you're done coating all of your cabochons, take a look to see if any need more resin or if
it has dripped off the sides. Where the resin has dripped off the sides you have two choices. One,
you can leave it be and cut/sand away the excess resin once it has set. Or two, you can
move that cabochon to a new, clean spot on the waxed paper being careful to avoid tilting the cabochon.
The later is trickier to achieve, but often easier in the long run.
If you want to topcoat both sides of your item, coat one side and let it set completely, before coating
Bubbles. After you've finished topcoating all of your cabochons look them over closely for air bubbles.
Many of these can be removed simply by breathing across the surface. If that doesn't work, I've used an embossing
gun, blowing it across the surface to cause the bubbles to pop.
Painter's tape surrounds broken concrete cabachon to contain epoxy
Stamped school of fish on concrete cabochon with resin topcoat - no bubbles!
Green cabochon from Part Two
with acrylic paint and resin topcoat
Clean Up and Curing
Epoxy resin is incredibly sticky and spreads everywhere it touches (rather like honey).
I've found that paper towels tend to spread the stickiness rather than removing it.
And soap and water are pretty much useless.
Instead I use wet wipes, or baby wipes. I used to buy the type with alcohol, but
none of the current brands available locally have that any more.
So I keep a bottle of rubbing alcohol handy, and dab a little onto my wet wipes as I work.
Use the alcohol treated wipes to clean your hands, work surface, tools, etc.
This works quite well.
Because I'm paranoid, I tend to leave all of my wipe up cloths in an open metal container over night, with
nothing flammable nearby.
Sometimes I have extra epoxy left over. If it's only a very small amount, I simply leave the cups out to set.
If there is more, then I'll either coat some painted papers (I often keep some small designs handy, just in case), or
simply pour the excess out onto waxed paper to dry. This way I don't worry about an unintended 'deep pour' curing in my
Put your cabochons in a level, out of the way place away from heat sources or direct sunlight and leave them overnight.
While many resins will set to the touch fairly quickly, they all take 24-48 hours to fully cure. Before handling your cabochons,
test the surface by tapping an extra bit of resin (there's almost always at least one drip on your waxed paper) with a finger nail.
If your nail leave a mark, everything along for a while longer. If no mark results, then the pieces are ready to be handled.
Once the surface has set, but while the epoxy is still pliable, you can use a utility knife to trim away any excess or overflow epoxy.
Once the epoxy has hardened completely it can be sanded. Using 300/600/900 grade wet/dry sandpaper you can achieve a soft,
matte, semi-transparent finish. After sanding, the easiest way to return the high gloss finish is to add another thin top coat.
If you used a rubber band or painter's tape, remove them as soon as possible after the epoxy cures. I sometimes need to use a utility knife
to get between the two layers, but can normally remove the barriers cleanly. Any detritus can be carefully removed using either a utility knife
or sandpaper as detailed above.
Envirotex has a fantastic tips and troubleshooting guide.