primer

I don't normally prime anything.  If have a situation in which I have a lot of putty or light colored scratch-built plastic parts on a dark plastic model, I might spray a coat of gray to help even out the colors before going with my actual color coat.

When I was kid, I used freezer tape because it was available, but it is not the best stuff to use.  3M Blue tape does a much better job at making clean lines and being lower-tack is a lot less likely to pull up underlying paint.  Even better is Tamiya's Yellow Tape.  It's a bit pricier, but it gives really clean lines, is flexible enough to conform to compound curves, and is low tack so it doesn't pull up the underlying paint.

Andy 

I'm not sure about freezer tape. I never used the stuff. If it is a low tac tape, it should work. However, The 3M blue painters tape works great. Make sure it is the "delicate surface" type. I use this kind exclusively.

Andy 

If you are just priming plastic, I normally just do it once as long as all surface are all evenly covered.

For PE and resin surface, I do it twice just incase.

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Ben