Setting Compounds require water to set hard. This chemical setting occurs after the specified time for a compound (i.e. a 90 minute set product, actually starts to set after 90 minutes). If the water content in the compound is lost or greatly reduced before the setting process starts, e.g. the compound has dried before it sets, then the chemical setting process can not occur and cure the compound to achieve its required strength and adhesion. The symptoms of this are the compound will be soft, have little or no mechanical strength and adhesion, and the tape may bubble, crack or even fall out. Setting compounds must not dry out before their specified set time.
Drying out before setting can occur due to a number of reasons, including:
longer set time compounds
inadequate compound placed under the tape
no compound placed over the tape immediately after bedding in of tape
compound is watered down or unauthorised retardants are added
To avoid drying out before settling, Winstone Wallboards recommends the following:
Do not use 150 minute set time compounds when the temperature exceeds 25-30°C (in front or behind the joint, e.g. skillion roof on a hot day can create very high temperatures behind a joint)
Use lower set time compounds, e.g. 90 minutes or less, duriong the summer months of November to April
Plaster ceiling joints located directly under the roofs early in the morning when the temperature is cooler, especially during summer
Be wary of areas where insulation has not been placed in ceilings e.g. garages, as the plasterboard will be hotter causing the compound to dry more quickly
Ensure at least 0.5mm of compound is placed behind the tape, use only paper tape, and coat the finished side of the tape with a thin layer of compound to slow evaporation under the tape
If machine tools are being used, ensure the compound is being applied at the correct thickness
Use back-blocking to significantly reduce the amount of load transferred to the joint
COLD AND DAMP WEATHER CONDITIONS
Water, wet compound, air and surface temperature above 10°C should be maintained until the compound is completely dry. Structures may require heating to achieve this temperature. Use of a 1 x 2KW fan heater per 50m2 of floor space in a house is usually sufficient. Do not raise temperature above 18degrees celsius as other issues may arise. During winter months or under cold damp conditions, drying rates can be improved using more but thinner coats of compound. Setting compounds dry quicker and are better suited in colder conditions as a first and second coat.
In winter there is increased risk of shrinkage during these colder months. If the air drying compound is not dry, it has not stopped shrinking.
Some air drying compounds are promoted as a total system, including the taping coat. These systems are common overseas in warm climates or houses that are heated during construction once closed in. Warm air allows the thick taping coat of the air drying compound to dry (and shrink) in a reasonable time.
In New Zealand however, for at least 6 months of the year, the temperature is too low to enable a commercially acceptable time frame for drying of a thick coat of air drying compound. Added to this, for cost reasons, most houses are not heated in winter during the finishing phases of construction.
It is unwise to use air drying compounds as a system, unless you are certain that each separate coat will dry, so that shrinkage is eliminated.
This cannot be achieved in winter without heating the building being stopped. Doing this would eliminate most of the subsequent issues seen by both the stopping and painting industry, provided the workmanship by both parties is up to scratch.
If the individual coats of a joint are not 100% dry prior to the next coat being placed, delayed shrinkage, caused by the moisture eventually drying out in warmer weather, will create hollow joints, shadow lines on joints, tram lines either side of the tape, etc.
A thin coat of air drying compound on top of a plaster based coat will dry quicker than a thick coat. And because it’s thin, it will shrink less in relation to the entire joint. This system generally works, however will also benefit if the house is heated.
In conclusion: think about the implications for total air drying systems before using them. They have their place in heated premises or in warm / hot climates. They should not be used as a total system in a building that is being stopped in winter without heating.
SHRINKAGE & DRYING
Setting compounds usually shrink back to a degree to ensure the joint tape is pulled back into the joint as it sets. Once set, the joint will continue to shrink at a much reduced rate as the compound dries, and stop shrinking when dry. Painting a joint that has not dried properly can delay shrinkage by several weeks or even months resulting in delayed shrinkage.
INSTALLING WALL TRIMS
When bedding in paper lined metal trims such as the GIB® GoldlineTM products or metal trims such as the GIB® Rondo® angles or the GIB Slimline angles, setting compound should be used. Stronger and more reliable adhesion will be achieved when using a setting compound.
Back-blocking plasterboard joints eliminate movement in the joint and thus greatly reducing the possibility of cracking, and popping and peaking in the joint area, if done correctly. Refer to the GIB® Site Guide where back-blocking is required. The back block must cover most of the joint, and must be stuck down properly. Use GIB Tradeset® , GIB MaxSet® or GIB-Cove® Bond as the bonding agent, and ensure it has been put on at least 3 mm thick. Compounds must not be applied if they have started to set, or are thickening up.
SKIM COATING AND RENDERING WITH VICTOR® MULTI PLUS
GIB Plus 4® is the recommended compound for skim coating. Use Victor® Multi Plus for skim coating or screeding/rendering a solid plastering over a masonry surface, (i.e. brick, block and tilt slab and in an interior environment). Victor® Multi Plus can be mixed with sand and applied up to 10 mm thick in one application. An unblended coat is usually applied as the finishing coat. This plaster compound differs from jointing compounds in that water can be applied during the setting stage to allow polishing of the compound to a very smooth "glass" finish. Trade level skills are required to achieve a flat polished surface with this product. For application details refer GIB® Site Guide. Victor® Multi Plus is available in a 90 minute set time and 20 kg bags.
LIMITATIONS AND RECOMMENDATIONS
Ensure that (partially) hardened compound is not mixed with fresh compound, and thus ensure tools etc are thoroughly cleaned when using a new batch of compound
Do not mix compounds
Add only clean water and only as per the instruction on the packaging
Store compounds as per the instructions on the packaging
GIB® compounds are for interior use only and after installation should not be subject to high levels of moisture
Generally a longer set time compound has a lower joint strength if everything else is kept the same. Adhesion in setting compounds is by mechanical entanglement with the surface of the board or joint tape. Do not apply setting compounds over air drying materials
Do not use setting compounds for skim coating walls, as a thin coating will dry out before setting and can cause issues with paint adhesion
Air drying compounds are normally not suitable for gaps or filling exceeding 4 mm or 1mm deep, as they take an excessive amount of time to dry out. A setting compound should be used in this situation.