Most fiberglass column manufacturers recommend and alkyd (oil ) based primer and paint. I consider this recommendation to be somewhat outdated.
The columns must be clean, and a light sanding with 120 grit is also recommended. You can then apply an alkyd oil based primer or a high end acrylic latex primer followed by two coats of high end acrylic latex paint. If you are using an alkyd oil finish paint use an alkyd oil primer, do not apply an alkyd exterior paint over acrylic exterior primer. However, you can apply acrylic latex paint over an alkyd oil primer.
I have seen bonding issues with columns that were not prepped and/or primed, IMO priming is necessary.
Best Luck
rmichael