Timber movement - Primer appearing under new paint
Timber is a natural building material that has an affinity with water. When it is first cut, it is absolutely saturated and must be dried out to be useful. Normally it is dried to about 17% moisture content. As the moisture dries out the timber also shrinks. If the timber dries out further it will shrink more and if it absorbs water it will swell.

In some ways it is similar to a sponge cloth that also shrinks when it is dry and visibly swells when it is wetted.

If a weatherboard is made and fixed during the cooler months of the year it is liable to contain water at the top end of the range. If that weatherboard has been painted with a paint, the colour of which absorbs heat; as the warmer months come, some drying out of the board can occur with the inevitable further shrinkage. That is when a tell-tale line of primer can become visible at the bottom of each weatherboard.

This problem is more likely to occur with dark colours and on exposed northern aspects. And the remedy? The only remedy is touching up with the original paint to give a uniform finish.