All hardwoods should be left uncoated for as long as possible, until all tannins have leached from the timber.
For oily timbers: Kwilia (Merbau), Tallowood, Blackbutt etc. A minimum of 3-6 Months is recommended.
For dense timbers: Ironbark, Spotted Gum, Yellow Balau, Batu, Ulin, Messmate etc. a minimum of 6 months is recommended. Ironbark may require significantly longer.
Rain/Moisture is required for the tannin to relase from the timber. For dry areas/drought the time required may be longer.
Tannins are usually visible on the surface of the timber as a brownish or tan discolouration. Tannins are an acid and will break down any product applied to the timber.
Hardwood Weathering Test
Apply some water to the hardwood timber. Let it sit for a few minutes. If the of water is absorbed into the timber, the timber should be ready for painting. If the water beads then the timber needs to be weathered for a further period.