"Metal Ready" is an interesting product. counter-intuitive but I"ve used it with good short term results (Long term we'll see).
I prefer Ospho for metal prep on rusted stuff, especially if I have time to leave it and the area is ventilated (no puddles). You just need to degrease your part, knock off the big stuff, spray ospho on, then come back in 24/48 hours and prime/paint.
Regarding por15... meh... it's over hyped in my opinion because people try to do too much with it, but it is a very good product if used fresh and applied per instructions.
You didn't mention getting the degreaser and that is key for a frame that's seen all types of dirt and junk. You really want to purple power / simple green or TSP-pressure wash all the grease off, then metal prep (phosphate rust converter), then POR15.. if you top coat it with a chassis black you should be set up.
I like rustoleum for frames, even though it is soft it is durable for that application, flexes well, and is cheap. Add the hardener for better results
POR15 does not flow like normal paint. It goes on thick and tacky like but spreads thin and will sag easy. Make sure you protect everything under the painted area because if brushing it will likely drip.
POR15 works by forming a water-impenetrable vapor barrier between steel and the air, preventing water molecules and air molecules from contacting the steel underneath and forming rust. the metal-etch actually chemically converts rust to a stable form, or etches / zinc coates clean metal and gives better adhesion. More coats of por15 will enhance it's function. If it flakes and air/water (or ICE!) are allowed to infiltrate it will bubble like any other chassis paint. POR15 works so good because it is very durable (resists chipping / scratching), has good adhesion and it is a good vapor barrier. Compromising prep effects adhesion. putting it on too thin effects the durability / vapor barrier.
I use a mixture of bar oil and wd-40 every fall in lieu of painting and it seems to keep rust at bay.