TOMPS Glue Size Plaster Retarder. Sold in 1, 2, 5, 10 and 25kg sacks. The traditional retarder used within the ornate plasterware industry. A little on the smelly side but easy to use and less likely to cause a problem. If you add too much size to your plaster mix it will still harden, it might take a long time but it will harden. Tri sodium can fully inhibit plaster hardening processes.
This mix may be too strong for your needs, if it is, split it in half and add an equal amount of hot water to both halves to achieve the right concentration. The only way to establish this is by running pot tests. You should always use the same pot to add size to water prior to adding plaster, this will help regulate set times. So pick a small pot and keep it for just this job, often this is attached to a stick/broom handle and left in the bucket for easy mixing.
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