Painting Shed Interior

Solvent based wood preservers like original Cuprinol) tend to sink in - so usually lighter shades don't cover darker ones particularly. However some of the modern water based wax loaded ones might since they tend to form a coating on the surface.

So either go for a solvent based one in a light colour, or fill all the board joints on the inside with decorators caulk or similar.

Or dod an insulation job inside and paint over that. Depends what its going to be used for. Brian

The nature of shiplap is that any internal drips are likely to find their way outside and stain the dark creosote. However, the good news is that you can then go over the dark creosote on the outside and touch it up, safe in the knowledge that any drips won't find their way inside.