Is wood primer necessary?
You may need primer for painting new and unfinished wood (some paints are all-in-one). New woods are typically thirsty, absorbing a lot of paint, and the grain creates variations which will leave an uneven surface for you to paint over. Wood primers help to create a smooth surface for undercoats and topcoats to adhere to, for the best finish.
Can I use white paint as a primer?
Only if you want to start the process from the beginning, costing more time and more money in materials and labour. Cheap white paint isn't a substitute for primer, lacking the ingredients and contents to create a paintable surface that will be durable.
Can you leave primers unpainted?
Primers are meant to be painted over, once dry, with a topcoat. Primer lacks protection from elements as they are not meant to withstand UV light, abrasion, etc, and will chalk and deteriorate over time.