Actually, some people insisit on PRIMING a model
if it contains any metal parts (see, for example, Tony
Greenland's book). It seems, though, that priming is
really necessary if you have metal parts that can
rust/corrode with age, for example lead foil.
You should be aware about what your particular
photoetch (PE) set is made of and how easily this
material can be corroded.
I've accidentally performed a little experiment on one of my
models that had lots of ABER PE stuff (this one is made of
brass which is a very common PE material) on it. I've washed
it with soapy water to remove finger oil and prepare it for
painting and then had to abandon it for nearly a month.
After getting back to it I noticed that my PE pieces had no
corrosion whatsoever and skipped the priming (which was
IMHO a good thing, I was afraid that primer will "thicken up"
some very delicate PE parts like engine exhaust meshes).
After airbrushing the model with Gunze acrylics I really
can not tell anymore which parts are plastic and which PE.
I would say that probably in most cases it should be safe to
paint over PE without priming. But if you want to be extra safe
or are using some offbeat metals in construction you should
prime first.
Agggrrrr.... Does this make any sense at all? :)
Serge