We have used new liners, brand new rolls of wire, and have obviously checked for gas leaks- (single bottle/regulator set ups in this case, no manifold). We have had consistant porosity with all of them. Also tried acetone only, and it didn't improve.
The other day, we were considering the possibility of the joint designs causing us problems because they are narrower due to the European design than what we are used to. For instance, a single V with 2mm root opening, 2mm face and only 50 degree included angle, or a single bevel tee joint with 2mm root opening, 2mm root face, and only a 40 degree included angle. So with the narrow angles, I was thinking it might cause turbulance as the gas has to "accelerate" of of the narrow joint. Possibly a verturi type thing. So we opened the joint up a little and also slowed the flow rate down to 25 CFH. We tried 2 tests that way and both of them still had a little porosity, but it was definatly reduced.
I was also working with QC to get the right interpretation of the film once I found out they were using a more stringent criteria than necessary. The Eurpean code we were able to dig out actually seems quite lenient with the amount of porosity allowed, although is isn't as easy to understand not having worked with it before. Once that was accomplished, we only had a couple of rejects instead of all of them. I still want to find out what is giving us the problem, but for right now, at least we are back in production.
As far as the porosity description, it is very uniformly distributed on the RT. In the break tests, we have found it in the root, fill and cap passes. Most of it is 1/32" or less in diameter, with about 10-20% being larger (but all but one or two being less than 3/32" in diameter).