The initial cure of the plaster (60%) takes place during the first 28 days. During this time, it is crucial not to "traumatize" plaster. Also during this time, you are most likely adding acid and brushing the pool frequently to promote an even cure. Basically, you want to give the plaster and water enough time to stabilize before adding salt. Adding salt to soon will throw everything off; TDS, Calcium levels, pH and Alkalinity, etc... This will often result in an uneven cure or discolorations as well as scale.