Check out this stuff, maybe a good option for you. I coated my entire frame with this stuff when I stripped it down and then sprayed frame paint over it when I was done
Permatex - Extend Rust Treatment (81849)
Provides one-step rust treatment that destroys old rust, prevents new rust. Just brush or spray it on. Within minutes, rust disappears and a durable black polymer coating develops. Excellent protective primer. Leaves a tough coating you can paint over. Specially formulated to be used with body filler and fiberglass.
Suggested Applications: Pipes, valves, fittings, truck trailers, storage tanks, fences, guardrails, conveyors, supports, overhead cranes, booms, floor gratings, ductwork, agricultural equipment, snow removal equipment, outdoor sign supports, posts, hatch covers