Like painting it's all down to the prep.
Was the metal de-rusted using electrolysis first ?, it doesn't matter what you put on rust, it will come back, electrolysis is the only way to rid the rust properly, it's piss easy, and costs less than £15 to get everything you need to do your own bits, to get rid of 100% of the rust, and that £15 even includes the cost of electric!!
Just buy a large container big enough to take your arms etc, some washing soda/sodium carbonate, and make a weak solution in the tank with this and water, hook up your battery charger on low and use 2 bits of metal bar one at each end of the tank, hook one end up to + and one to - and put the metal bars in the tank, switch on and leave for a few days, all the rust will go, all of it.
If this is done prior to painting/powdering you will get a much better long lasting finish.
Powder coating is piss easy, 10 mins at 180C for most components, you can apply more heat or for longer to cure, a "oven can be a simple cardboard box with a fan heater pointed at it, seriously there is nothing hard or magic about powder coating, just somewhere big /hot enough to set off the powder, done very carefully you can also mix powders for different colour schemes like striped etc, but this IS seriously hard to get right.
All you need for powder coating is a small lecky supply, air supply about 15 psi, plus the gun.
There are various diy kits on the market but one I have used to show someone else, and can vouch for is www.electrostaticmagic.co.uk
I did powder coating for a major supplier of double glazed window frames for around 3 years, it is a huge amount easier than wet painting, and very hard to get wrong.
tabetha