If you plan on parking a build you need to prep some of the parts before assembling. grease and oil will ooze out in the park bath contaminating the solution and possibly messing up the part. So it is a good idea to degrease the trunion and trigger guard and center support if you are using a flat before assembly. This will also keep your cabinet and blast media clean. I use acetone to clean the parts by soaking them. If you have a completed receiver you can boil it in a solution of ZEP Orange degreaser and water, followed by boiling plain water.
You are now ready to blast.
There are various types of media to use. glass beads peen and do not cut so they are not the best choice. Also available is ground glass, I haven't tried this but is should cut better than beads.
The next option is blasting sand, this is sand that has been size graded for blasting, it is very abrasive, cheap, and will do a good job. the only draw back is it breaks down with use and needs to be changed more often than aluminum oxide which is the best, it runs about $40.00 for a 50lb bag and if you dont contaminate it it will last a long time, blasting sand is about $7.00 for 50 lb.
I got a blast cabinet from HF for $69.00 it is perfect for this aplication. I spent another $15.00 and installed a flourescent light to improve visibility.
When you blast your parts it is important to blast from several directions around the rivets and other parts to prevent shadows, these are areas that are not properly blasted and will not parkerize properly. take your time when blasting and when it looks done go over it one more time. after blasting use clean gloves or a wire to handle the parts. do not touch them with your bare hands from this point on.
Now you are ready for pre black