STEP 4: Carving Touchups
So now you have your basic rock structure carved. Is the time to give it a last once over. Use your finer penknife to add small details to the Styrofoam like surface indentations and rock imperfections. The more the merrier, but remember that details that are carved too fine will be lost when concrete is applied. You may also want to lightly sand any excessively smooth surfaces with rough sandpaper to aide in concrete adhesion.
Finally, use your shop-vacuum to suck up any Styrofoam particles/bits from the background as they will only cause trouble when you begin to apply the concrete.
STEP 5: Applying Your Concrete
I won't get into the specifics of this as it is well outlined in the other articles on this site… but I started with one thin primer coat, followed by 4 more thin coats… allowing each coat to dry in turn. Using many thin coats is preferable to a couple of thick ones as it allows you to maximize the level of detail, important in a small tank.
You will have to play with the amount of water that you add to get the desired consistency. Too little water and it won't paint on well (papa bear), too much water and it will dribble all over the place like soup (mama bear). You want something in the middle, you are striving for baby bear. For every layer I used only a brush… never my hands. It might work for much larger backgrounds, but I will repeat, with a small tank detail is key, and using your hands results in loss of control, especially around cracks. You don't want to completely fill in what you spent so much time carving. Dabbing with the tip of your brush also helps to create a nice rough texture for the concrete.
In some of the finer nooks and crannies I used the smaller brush to apply the concrete. Again, be careful to not fill in all your carved detail. In the end you want to maximize detail without sacrificing concrete sturdiness/thickness. Do not be skimpy on the concrete or you will regret it down the road if it cracks and starts showing bare Styrofoam.
A note on drying: Slow drying is key!!! If the concrete is allowed to dry too fast it will crack and possibly flake off in the near future. In past articles it was mentioned that cracking could be avoided if you spritzed your drying background with water 3-6 times a day. I will admit that I am too lazy for that, so I just opted to cover the top of my tank with plastic wrap instead. Just put the plastic wrap over the entire tank top, tape it down tight, and then poke some air holes in it to allow moisture to slowly escape. With 6-8 pencil holes it took each of my layers about 4-8 days to dry, which was perfect. Putting you background in a large Tupperwear bin with a lid will also work to slow the drying process. I did this for concrete coats that were applied before siliconing the structure in the tank.
STEP 6: Silicone Your Background to the Glass
The moment of truth!!! Gob on tons of silicone to the back and bottom parts of your background and put it in. There is no turning back now ;) Be sure that your background/tank is 100% dry and clean when you do this step or adhesion will be affected. Glue down any Styrofoam rocks, and glue on any rock overhangs. When things are completely dry you may now want to do some touch ups with the concrete mix.
In my case I did my first 3 coats of concrete on my background (drying it in a large covered Tupperware bin), siliconed it in the tank, and then did the final two finish coats (using plastic over the top to slow the drying). This was a bad idea in the end though… because my big 3D overhangs combined with the small tank size resulted in me having to twist my arms in some pretty weird ways to apply concrete in some areas. Of note, I chose not to use concrete stains as they were expensive, and I was not sold on how good they would look… besides, algae makes a great natural stain.
Concrete on the bottom: In the end I had some extra mixed cement so I decided to use it to cover the bottom glass of the tank. Why? Well I was planning on keeping shell dwelling species such as multies in my tanks and they like to dig in the sand so much that they often expose glass. I hate seeing glass in my tank (reminds me that it is a fish tank, not part of the lake) so this way they can dig all they want, but all they will expose is concrete.