pad skipping after HASL

Dear forum types, 1st timer so be good. I have a customer in China using Coates soldermask that in its self is not a problem. The problem is the flux their using, I think,it appears to have a vary high viscosity. The problem they have laid at my door is soldermask blistering. The cause of this is they are having to hot air level some panels 3-4 times to get some pads to take. The solder dams of 50 micron are blistering and can be removed by tape testing. I have checked, and so have they, but there appears to be no soldermask residue on the pads, this was my first thought but apparently not the right one. So I womdered if the viscoity of the flux could be an issue. I also ran some panels down the ENIG line they were fine!! Any ideas? Thanks Stuart

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