I use 2 Fien vacs on my 10 zone 96x60 table. I've never been able to find Trupan, but have had good results with Sierra Pine UL. I tack glue the spoilboard to the plenum that's painted with epoxy paint. My normal procedure is to use the 3/4 UL done to +-1/8 then surface it down to the plenum, re-glue and start over. That's the history, now the problem...
My most recent UL replacement is not letting much air at all through. I've tried 6 different pieces, I've tried 1/2, and I've even laid different grades and thickness of plywood on the table, but no matter what I try the gauge reads 4-5 inches with nothing on top of the spoilboard. I've flushed the plumbing and cleaned the vacs. Most of my work is on the 2x2 sections of the plenum, so I've got two vacs pulling on 1 section of 2x2 getting nearly 5 in with just UL on the table.
I'm not sure why this method has quit working all the sudden, but I'm running out of things to try. Anyone else experience anything like this or have any advice would be a huge help.