Painting MDF Doors: Factory Primed…Primer? What Primer?
Our basic rule for painting mdf doors that were primed at the factory is to pretend that they were not primed at all. Do whatever cosmetic tweaking is needed, and then start building your finish as if you were working over raw substrate.
The factory primer is really intended to do nothing more than seal the door for dimensional stability from the time it is assembled to the time it is delivered to the field. It is a cheap, thin, chalky primer. It’s adhesion is adequate, but it is not suitable as the base coat for a professional finish because it lacks body. It is easy to break through and expose raw mdf substrate. It is best to just apply your own prime coat than to attempt feathering or blending spot priming into this junk.
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Waterborne primers have proven to be ideal to apply over the factory primer because they dry quickly, and can be sanded without concern for breaking through. They make a great base coat for waterborne topcoat adhesion as well.