I blowout my sprinkler system, configured very similar to yours, with a 3HP / 12 gallon compressor. Ideally, you'd have a 25 gallon (or larger) tank, but it is not necessary. Do each zone at least twice, starting with a fully charged tank each time - being a little paranoid about my self-installed system, I do each zone three times. Be sure you don't force excess pressure into the system. 45-50 PSI should be OK on the "big" zones, go with 30 on the zone with little heads.