Winterize Sprinkler, where to hook up Air Compressor?

Winter is coming so the ritual starts with winterization of the Sprinkler System.

This is a diagram what I have now:
- Supply from the house into the Backflow Preventer with the standard 2 testcocks.
- Hose Bib upstream of Backflow Preventer.
- And no, I don't have a winterization port downstream of the Backflow Preventer (I understand this is the best place to blow compressed air through!).

For the last 15 years, the Sprinkler Co. always inserts the air hose into Testcock #2 and it works fine.

I have read in many websites that people should not blow compressed air through the Backflow Preventer for fear that compressed air may damage the internal parts of the Backflow Preventer.

So my questions are:

1. Blowing through the testcocks:
Is it an acceptable method when you don't have a port downstream of the Backflow Preventer?

2. I have an adaptor that allows me to blow through the Hose Bib (#3 in pic), this is almost the same as blowing through the testcock because compressed air travels through the Backflow Preventer anyway.

3. Has anyone had a damage to your Backflow Preventer from compressed air?

Thanks!

I always run through the Testcock (#2 in your picture) with the Ball valve clsed between # 2 and #3 to isolate the irrigation system and it works fine.

Always use low proessure ( I run 25 PSI ) takes a litttle longer but I do it because I have a lot of drip circuits. I then run my controller for 10 minutes per zone.