Hairline cracks are caused by the expansion or contraction of wall surfaces. These wall cracks usually happen on plaster or skim coat surfaces, and are usually shallow, not more than 0.5mm deep. While wall cracks are an aesthetic problem, they may also be a sign of structural issues. A professional should be contacted if your house has wide or long cracks, but generally, hairline cracks can be fixed with the right products and surface preparation.