This can be caused by dust and dirt in the air settling on the paint surface; by using a dirty paint brush, or by stirring skin into the paint. In either case, let the paint dry and harden, then rub down using wet and dry abrasive paper and warm water with a bit of detergent. Rinse thoroughly with clean water and allow it to dry before repainting. Use only good quality paint brushes and wash out with a little turpentine, making sure the brush is completely dry before starting work. Avoid stirring skin into the paint by straining before use.