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HOW TO MAKE FAKE ROCK
May 20th, 2010 | 
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Many methods can be used to make fake rock and boulders and all achieve realistic results.The method you choose to make fake rock is often determined on the price of the filler material and availability. Very realistic faux rocks or boulders can be made using any of the following filler materials.
Building Materials and Methods
- Building rubble such as second hand bricks or crushed concrete.
- Recycled real rocks
- Polystyrene
Alternate methods to make fake rock involve using some way of giving an approximate shape to the finished rock before the application of the mortar.
- Chicken wire bent to shape is widely used for the smaller rocks
- The general shape can be formed with carboard boxes cut to size.
- Rebar and expanded metal.
If the same design of rock needs to be repeated many times the use of moulds will greatly assist.
- Casting from a mold
The material and method you use to make fake rock will be the one that you consider to be the fastest and cheapest to give you the best finished result. The level of detail in the rock usually determines the amount of time needed to complete the job.
Garden boulders and other larger rocks usually look best with a smoother and less detailed finish compared to smaller decorative rocks and require less work and material.
A garden landscaping job will often require the use of several of these different finishes to give the required natural look. The more methods and techniques you know how to use to make fake rock the better the final result will be.
Solid Fill Method
Due to readily available building material on site this is one of the most widely used methods but it is also slow and expensive. Not being of hollow construction it also gives the closest sound and feel to natural rock.
Materials which are required for this method include:
Rubble from demolished buildings.
This material can usually be obtain free of charge as it is most likely destined for disposal. It is essential to hose it down before use to ensure proper bonding.
Concrete blocks and recycled bricks
It is still cheaper to use these materials rather than concrete even if you have to purchase them. Difficult shapes such as waterfalls are also easier to construct due to the regularity of their shape.
Polystyrene
As a by-product from packaging this material can usually be found free of charge. The nature of the product allows it to be easily shaped and produces quite a light weight finished faux rock. It holds the shape of the rock and does not slump with the weight of the mortar, unlike some of the other filler materials.
Recycled rock and other natural materials
These are often on site and easy to use giving very natural shapes. Wash before use.
- If the material being used is in small pieces it can be placed in plastic bags and stacked for ease of assembly.
- Cut open the plastic bags to expose the filler material inside.
- You will then need to apply two coats of acrylic polymer stabilizer. This should be thrown on using a brush and is designed to ensure the structure is rigid before applying the finishing mortar mix.
- Now apply two coats of the mortar, with each coat being 40mm thick.
- Complete the process as described with the other techniques.
This type of faux rock is relatively easy to make. It also uses plain concrete and a basic mortar mix for the final shaping to further save any complications. You first need to build up a base for the rock in concrete, then build up the shape a bit with some rubble, add more concrete, more rubble again and so on until you have achieved the finished shape.
To make fake rock with this method it is a good idea to be doing several rocks at the same time. As the concrete sets on one piece of rock you can be creating another piece. Application of mortar can commence once you have completed the rough shape and it has set.
The mortar generally adheres better when it is applied in a throwing on action rather than trowelling. It also provides a rougher surface for successive layers or mortar to be applied to. Start from the top and work downwards with the finishing coats. If you are working on rocks which are side by side take care to ensure join lines are emphasised. Definition is very important with these seams to give the rocks a natural appearance.
Construction methods for hollow rock
Chicken Wire
This method is suitable for making hollow rocks of a smaller size.
- Cut and wrap the chicken wire to the rough shape of the finished rock.
- Bend all sharp edges of wire inwards using pliers.
- Now cover the wire with some newspaper soaked in 50/50 diluted white woodworking glue and allow to dry.
- Cover the wire with several layers of plaster of paris bandage which has been briefly soaked in water and allow to dry.
- Using diluted acrylic paint mixed 50/50 with sand decorate the final rock to create a realistic stone representation.
Re-bar and Expanded Metal Construction
If you need to make fake rock which is hollow this is a very common method. Using this method rocks can be shaped to cover taps, pool pumps and other equipment. This method can produce waterfalls to garden arches due to its ability to be easily shaped.
The expanded metal can be attached once the re-bar has been set up. Tie it to the re-bar every 100 to 150mm. Ensure that it is firm. Coat the structure with two 3 to 4cm coats of mortar mix leaving a minimum of an hour or overnight between coats, depending on the temperature of the day. Add texture to the project using similar methods to the solid fill method.
Cardboard
Large and small cardboard boxes cut to shape can be built up to form a strong structure resembling the desired rock shape. Making faux rocks of all shapes and sizes is easy using boxes that are readily available free from your local stores.
- Make the structure rigid by stapling, wiring, screwing or gluing the cut box shapes together to form the rough outline of the desired rock shape.
- Using a throwing action with the brush, apply two coats of polymer stabilizer mix straight onto the boxes. If too much is applied at once there is a danger that the cardboard will be soaked and collapse. Between coats allow the cardboard to full dry allowing it to attain the required strength to support the final mortar mix.
- Two coats of mortar mix can now be applied in 8-mm thicknesses.
- Now you can texture the structure followed by staining and sealing.
Casting from a Mould.
Using a mould can produce the most accurate representation from natural stone of all the methods. They are perfect reproductions of stones in nature. The same design of stone can be reproduced endless times. Moulds can be made using latex or silicone usually with a plaster back up mould to retain the original shape.
The process which involves the use of texture moulds will be the subject of a forthcoming article.
Copyright © 2010 Stan Alderson
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Question on Mould Release Agents
Question:
Hi. I have recently purchased a business making concrete statues with latex moulds. I have ran out of mould release agent and not sure exactly what it was we were given with the business and the previous owners couldn’t remember where they got it from either. Do you guys sell any mould release suitable for this purpose? Your help will be greatly appreciated.
Cheers,
Valued Costumer
Answer:
Hi,
Light grade machine oil is the most effective release. However the plastic and rubber manufacturers warn us that continued use of a petroleum based product causes a breakdown over time of the plastics and rubbers used in the concrete industry. For this reason we recommend that a vegetable oil be used. Industrial castor oil is best but peanut, canola or other oils work quite well too.
The thinner the viscosity of the oil the better as heavy oils seem to retain air and yield air bubbles on the surface of the concrete. Wipe the mould out with a T- shirt type of material to leave only a thin film of oil in the mould. Excess oil clinging to the surface of the concrete after curing makes painting or antiquing almost impossible.
More information is available on our concrete mould website www.moulds4concrete.com.au
Kind regards
Aldax Moulds
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Question on the Right Mix Used on Moulds
Question:
Do you need a vibrating table to use this mould?
Do you have a sample of the mix that is used on this mould?
Kind Regards
Valued Costumer
Answer:
Hi,
In our mould instructions we recommend sand and cement mix for the smaller pavers. But the use of bags of ready mix sand and cement also gives good results. For larger pavers ready mix containing sand cement and aggregate is to be preferred for added strength. Pour the mix into the mould until 1/3 full and then bounce the mould against your bench to remove all the air bubbles. Continue this process until the mould is full.
Paver manufacturers use mechanical vibrators to achieve the same effect. The bouncing or wrapping action in addition to removing air bubbles, moves the larger pieces of gravel aggregate to the center of the mould. The finer parts of the mix such as the sand go down into the design of the mould. Properly vibrated moulds will show no sign of rough aggregate on the surface of the finished concrete paver, thus spoiling its appearance.
Kind regards
Aldax Moulds
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Mosaic Moulds
We have just added 7 moulds which will be suitable for making concrete mosaic pavers and table tops of various sizes. The mosaics are to be made by the indirect method which enables all pavers etc to be finished with a completely flat and even surface, essential for this type of mosaic.
Shortly we will be offering a range of glass tiles suitable for finishing the mosaic patterns which we also have available. These moulds are an interesting addition to our range in that they can also be used for making plain pavers or tiles by varying the pouring depth and colour oxides used.
To keep the price down we are making them with 1 mm plastic which provides a sturdy useable mould suitable for hobby or low production home use. All moulds will come with instructions and later this year in kit form.
Stan Alderson
6th October 2008
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How to Colour Pavers
Question:
How do I colour the pavers that I am producing with the ABS plastic moulds?
Answer:
The paver itself can be coloured by adding a metallic oxide base colour to the concrete mix before placing it in the mould. Additional colour or colours can be sprinkled into the mould before casting through a sieve to give a contrasting face colour to the base colour of the paver.
Stan
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Colouring Plaster and Concrete
Question:
Hi there, I’ve got a question. A guy up at bellingen uses some sort of color to color plaster like a sandy color. He says he dips it in. I can get you a pic of an item if you want to see one just curious if u have any ideas and other colors as they look pretty good and i got a lot of wall plaques i wouldnt mind doing.
Answer:
Hi, You get the colours in concrete and plaster by adding metallic oxides. We just put the colours up on our web store where you can see them. Click here to see the range.
All the best for Christmas.
Joseph