Danny, there are two different types of primer. You would not use use the material often referred to as "primer surfacer" which is a "build" product to fill in surface defects. It has no weather resistance.
Pasted below is a post I made on BNO last December. I think the information is still good.
The choice of primer is very important. I would not use anything less
than two part epoxy. I used PPG DP48 which is a white primer with a
slight sheen to it. Looks pretty good by itself. There are primers
that are used to build depth (primer/surfacer) and they are NOT
(repeat NOT) protective coatings. They will rust in no time.
The PPG DP line has been my choice for many years and has always been
called the super glue of paints. With reasonable preparation, it WILL
stick forever and not permit any rust. It was often used UNDER bondo
by the better shops to make sure the bondo stuck and that it would not
come off due to rust (bondo will let water pass through if there is a
slight crack in the cover material).
PPG DP48 (there are other numbers for other colors) costs about $100
plus the catalyst and thinner. In my opinion, it is money well spent
for the protection.
Now for the sad news (at least for me), it appears that PPG has
recently discontinued production of this paint. I have used it for
over 20 years and don’t know how I can get by without it. I just
bought some DP74 (red) less than six months ago, so the decision is
recent. I will make a quick trip to the paint store tomorrow to see if
I can find some. I did find a site that seems to have the product at
about half the price I paid (with supposedly a good discount): http://www.midwestchemicals.com/DP48LF-DP402LF.htm
It looks like Dupont 2910 is about the same material.
One good wet coat should give you good protection and should look
good. Be very careful to wear breathing protection this is some bad
stuff to breath.
If you use DP48 (and probably any of the two part epoxy primers), you
will need to sand it and recoat it before you apply any
primer/surfacer or top coat if it sits for more that a few weeks.