Davey, often in the beds you will find what is called "two part epoxy" which is hard but easy to work with because it is very forgiving in that it is thicker than standard paint and that makes it easy to remove sanding marks etc.

Epoxy primers are used a lot in the prep for paint stage because when you spray it on it levels its self and will stick to virtually any clean surface.

I like epoxy primer and would reccomend it for bare metal as a first coat.

I think you are wise to start inside the bed box to gain experience.

Hobert

"The Lord is my shepherd"