Exposing new dry wood to rain before painting is an unacceptable practice.

In tests carried out on wooden window frames on the Hanger Hill Garden Estate houses, paint that had been laid on damp wood during rainy periods lasted less than 2 years whilst on wood that was completely dry, the paint lasted over 7 years on average. See also Problems with the 2019 HHGE External repairs 03

These new hardwood cills were installed at Essex House on 7 July 2020 and then left exposed to the weather .
Photos taken 9 July 2020. Note that the cill on the RHS (orange arrow) is still dry but the cill on the LHS (blue arrow) is absorbing rainwater hence the difference in colour. As water has now been absorbed into the wood this timber MUST NOT BE PAINTED until both the interior and the surface are fully dry. In bright sunny weather with no rain, drying out could take several weeks. If the wood is not fully dry before an oil based paint such as Dulux is used then neither the paint nor the timber will last as long as they should.
At least 3 meters of good quality hardwood have been used for these cill replacements. This wood is called Eucalyptus grandis. E,grandis is an Australian tree, plantation grown in Paraguay. At a typical builders merchant such as Travis Perkins this timber would have cost over £120.00 Why pay for very expensive materials and then fail to treat them properly ?

See this link Problems with the 2019 HHGE External repairs 03