I used a heat gun to do some tricky spots on my packrafts. I used the “858D Heat Gun” (search for it on AliExpress). I flattened the smallest nozzle slightly with pliers. This allowed me to direct the airflow much more precisely onto the seams. I don’t recall the exact settings (temperature and airflow) right now, but if you need me to look them up, let me know and I’ll check. Oh and we have 220v here in germany. Therefore for 110v, the settings may need to be adjusted slightly.