What are rendering additives and why to use them.

Additives are liquid or powder ingredients normally used in render mixes that are cement based and should be measured carefully using the manufacturers instructions.There are different types of additives that can be used in cement based render;

Plasticiser is used to make the application easier.
Water-proofer is used to equalise suction and it creates a waterproof barrier.
Retarder is used in warmer environments to slow down the setting time.
Accelerator is used in colder environments to speed up the setting time.

Plasticizers are used in plastering mixes to make them easier to work with. They make air bubbles which stop friction between sand and other aggregates. The bubbles are still in the mix after it has set. This can help stop moisture build up. The air bubbles can sometimes be an advantage for when water expands in freezing temperatures. There are different types of plasticizers and others are used as a greasy lubricator to prevent friction between aggregates.

Bonding Adhesives are like glue because they stick to smooth surfaces and also plaster coats. Most adhesives are made of PVAC (polyvinyl acetate) but some are from polymers which have acrylic resins and styrene-butadiene rubber in. Styrene-butadiene does not react to moisture and is used most for external bonding adhesives. When applying bonding adhesives, you must read the manufacturers instructions carefully because the application process will be different.

There are three methods to apply bonding adhesive; brush, stipple and key coat.
Brush- for this you need apply a priming coat mixed with water, and then apply a priming that is not mixed with water. then apply the plaster while the adhesive is still wet.
Stipple - for this the adhesive needs to be mixed with cement and a small amount of sharp sand. The cement is stippled to make a key for the plaster to stick to.
Key coat - this is applied to the surface and a notch trail is used to make a key for the plaster to stick to.

Bitumen style solution is a different kind of adhesive. It is waterproof so can be used in areas where damp would normally be a problem. It creates the base for gypsum plaster to be applied on smooth surfaces. The solution should be applied to the surface with a brush. Once it has been applied, dry sand should be put on it while it is still wet. 24 hours is the recommended time you should wait before applying the floating coat.

Glass fibres are put in some plaster and sand/cement mixtures. This is to make the mix stronger. Glass fibres are chopped into strips and added to rendering materials when they are being manufactured with either white Portland cement r ordinary Portland cement.

With gypsum plaster mixes, special chopped up strips of glass fibre are added. These mixes are a lot stronger than haired plaster mixes.