I only used PERMA-BLUE one time and had excellent results. I refinished an old Ithaca shotgun top to bottom and it looked just like a $400 rust blue job I had done on a similiar gun. The trick to using Perma Blue is to take it one step at a time, do the prep work...for an AK receiver I would reccomend a 5gal bucket of water close at hand. Lots of polishing grade steel wool.
1. Take the receiver and completely steel wool polish it all over.
2. Get a small sponge and apply perma blue constantly rubbing it in...do not be afraid to get messy with it, soak the metal.
3. Dunk the receiver in water bucket and lightly polish with steel wool.
4. Wipe with a towel and repeat....20-30x.
5. Last time polish then lube it up with gun oil and let it sit completely soaking overnight in oil.
I know this sounds like a lot of work, but I spent maybe $25 in supplies and got an incredible finished product for my time. I endorse the stuff, based upon the gun I did have turned quite a few people on to the stuff after seeing my results.
Trust me, I also doubted any of those over the counter products....but this stuff works if you do your part.
Good Luck