Case 1

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Good Practices

Plaster

Material

The plaster mix proportions could be according to the Table 1 to minimize excessive shrinkage [3-4].

Mix	Mix proportions by volume based on damp sand
	Cement : Lime : Sand 1)	Cement : Ready-mixed lime/sand 1)		Cement: sand 1) (using plasticizer)	Masonry cement: sand 1)
	Cement : Lime : Sand 1)	Ready-mixed lime:sand	Cement: ready-mixed material	Cement: sand 1) (using plasticizer)	Masonry cement: sand 1)
I	1 : ¼ : 3	1:12	1 : ¼ : 3	–	–
II	1 : ½ : 4 to 4 ½	1:12	1 : ½ : 4 to 4 ½	1 : 3 to 4	1 : 2 ½ to 3 ½
III	1 : 1 : 5 to 6	1:12	1 : 1 : 5 to 6	1:5 to 6	1 : 4 to 5
IV	1 : 2 : 8 to 9	1:12	1 : 2 : 8 to 9	1:7 to 8	1 : 5 ½ to 6 ½
V	1 : 3 : 10 to 12	1:12	1 : 3 : 10 to 12	–	–
1) With fine or poorly graded sands, a lower volume of sand should be used.Note: In special circumstance, e.g. where soluble salts in the background are likely to cause problems, mixes based on sulphate-resisting Portland cement should be employed.

Table 1: Properties of plaster mix (Source: BS 5262: 1991)

The use of properly graded sand, in particularly the avoidance of an excessive proportion of a very fine material. Alternatively, ready mix plaster can be used to overcome situations like unavailability of the recommended sand.

Construction

Following practices can be followed-up to minimize crazing during construction

For background with a high absorption rate, dampening of the background before and during the application of the plastering in hot and dry conditions is essential.
The pre-packed skimming materials shall be mechanically mixed with the amount of water as specified by the manufacturer.
Avoid overworking, which causes an excess laitance to be drawn to the surface.
To prevent crazing, start curing the plaster as soon as possible. Keep the surface wet by either flooding with water, covering it with damp burlap and keeping it continuously moist for a minimum of 3 days, or spraying the surface with a liquid-membrane curing compound. Avoid alternate wetting and drying of concrete surfaces at an early stage. Curing retains the moisture required for proper reaction of cement with water, called hydration.

Quality Control

Avoid followings during plastering to minimize crazing

Do not dust any cement onto the surface to absorb bleed water
Do not sprinkle water on the surface while finishing
Do not trowel excessively as it will draw the water to the surface

Painting

Construction

Prior to the application or re-application of paint system, surface condition should be checked. Surface condition for painting process should be as follows[5-7]:

For new plaster, the plaster surface should be allowed about 30 days for proper curing and the reduction of moisture content. Further, it will need to be primed. The type of primer used will depend upon the final decoration required. If you plan to use oil-based paint, an alkali-resisting primer should be used. For water-based paints, a diluted first coat can be used to prime.
For old surface, the preparation required will depend upon the condition of the surface. If it is clean, dry, and sound it will require no special preparation. However, if the surface is powdery or there is other evidence of old distemper residue, it should be dusted off and sealed with a stabilizing solution.
For the painted surfaces, the existing paintwork should be washed down first. This is necessary to remove traces of dirt and grease, which would otherwise cause failure of subsequent decoration.

Quality Control

Quality assurance can be carried out as below:

STAGES OF WORK	CHECK	CRITERIA
Recognition of environmental conditions	Temperature of site	Ambient temperature > 5 °C or refer to manufacturer’s recommendations
	Substrate temperature	Surface temperature of substrate < 60 °C or refer to manufacturer’s recommendations Surface temperature of substrate > 3 °C + ambient temperature or refer to manufacturer’s recommendations
	Relative humidity	Refer to manufacturer’s recommendations
Checking of Surface Preparation	Method of surface preparation	Comply to approved submission
Checking of Surface Preparation	Prepared surface	Free from foreign and unstable matters such as dust, dirt, rust, cement, spatter, salt, grease and oil
Mixing of paint	Mixing ratio	Comply to specification Sufficient stirring until uniformity
Painting control	Surface just before painting	Painting should start preferably within 24 hours after preparation
	Protection to elements not to be coated	Mask or cover those elements not to be coated
	Process of application	Comply to approved submission
	Intervals between coats	Refer to manufacturer’s recommendations
	Wet film thickness	Refer to manufacturer’s recommendations

For more information on paint crazing due to moisture, please refer to Paint Blistering.