if you remove the paint to the metal, you'll need to use an etch primer for best adhesion, or at least one designed for metal. build (or filler) primer covers finish irregularites so you can sand them smooth afterward, but for lugged frames it will just fill in the lug joins, which you don't want. however powdercoat will do the same. if it's not yet stripped to metal you should leave the paint— it's already sticking to the frame metal.

take your time, wetsand and tack cloth between coats, wear nitrile gloves so finger oils don't foul the paint, spray using catalyzed auto paint if possible— or take to a professional painter once it's stripped.