"David Lang" wrote in message
k...

The primer is to give good adhesion to the material. Hence
different ones
for wood and metal, etc. Undercoat at one time was always near the
same
colour as the top coat but less expensive, so made the job
slightly
cheaper than using two or more coats of top to get the depth of
colour.
But things are a bit blurred these days. ;-)

What Dave said.

The need to achieve a good bond is still with us, but modern paints
have
such good covering power they make undercoat almost superfluous.


Depends on what you are painting, some surfaces still require the use
of a primer and then undercoat.