The air sucked through a compressor can be quite damp and if you are painting in a humid area this can have a negative effect on the performance of the paint. Most compressors that can reach a suitable pressure will have an internal filter that takes out water from the inlet air. If humidity is high then this is critical, whilst if it is low then it is less important. As best practice and to avoid the risk we always recommend use of a compressor with a water filter.