Hi Ykang
I know titanium is prone to this problem. I'm hoping some of our members with more day to day production experience of coating titanium will comment.
Factors to consider in reducing grit entrapment:
Use minimum of blasting to achieve the desired etch. Over blasting will increase grit retention, apart from being wasteful.
Blast pressure is critical. Too high will make the problem worse, too low will be inefficient. Blast nozzle type/bore size is also important aspect here.
Blast angle is generally reported to produce higher grit retention at 90 degrees than at lower angles like 55, but tensile bond strength tests indicate higher values at 90 degrees. Also the angle of thermal spray relative to grit blast angle can effect bond strength. Generally using the same direction for both produces best results down to around 55 degrees. Using opposing angles say 55 degree blast and spraying 55 degree from opposite direction though would not be good
Blast media - try to use only fresh/new, avoid spent and broken down media containing fines and contaminants. I know some like to use the white higher purity alumina grit, but I think this tends to break down quicker than the tougher brown versions containing titanium dioxide/silica. I've seen silicon carbide grit used, though I don't really know how effective it is, but may be worth a try. Grit size and particle distribution size range will be other factors.
I've heard some go to the extremes of ultra-sonic cleaning to remove some retained grit. I don't know if this is effective and there is always an increased risk of contamination and deactivation of the surface.
I can only advise that you experiment with your grit blasting procedure, bearing in mind the above comments.
Good luck