so i've done another wax with swix blue just now and the problem has stopped
I put on an extra 3rd more wax than I usually use and I moved the iron quicker across the base on a temp of 130 centigrade.
I was melting cold wax on so I timed how long I moved the iron the full length of the ski (181 antidogma). It took exactly 1 minute. that was approx 20 seconds faster than I usually do. I couldn't go any quicker otherwise the wax wouldn't melt properly.
I think the problem was that a mixture of not enough wax and moving the iron not quickly enough.
The bases never bubbled or the underneath of the ski never felt too hot, so I don't think I have done any lasting damage.
here are some before and after pics -