New traps I boil in degreaser / detergent or lye. I dip in dilute muratic acid to neutrallize the "soaps". Now I don't live in an area with much humidity, therefore I soak the traps in Chlorox and salt or dilute muratic acid and salt. This soak releases the mill slag form the steel ( the hard black surface coating). I hang the traps, rinse good with a hose and they rust hard in a couple days time. Please note I said dilute muratic acid 1 cup per 3 too 5 gals. This concentration won't harm you, short of getting it in your eyes; you should be wearing glasses.
If you live in an area with humidity your traps should rust quickly once you have removed the oil. However, I've found that I don't get a good pitting rust until the mill slag is off. An even rust opens the surface of the metal and the dip binds much better. I use the highest dilution of dip recomended and I dip on hot days to get a even thin coating.
For old traps I boil in lye or detergent rinse and hang.