hey everyone...first post.
had a glide decoy kayak for a couple of years now up in north east vic, and really enjoy getting out in it! just replacing my original FF, which was a lowrance, and replacing with the Garmin 4. My lowrance trnsducer was siliconed in -hull. when i removed it the other day i was surpriser by the amount of air bubbles in the goo, even though i was very careful in prep etc. i was never really happy with the FF and i wonder if these bubbles added to the problem. anyway, new start and i cant make up my mind whether to go in-lull of scupper hole. if in hull what have you found is the best method that works for you? Some of the american youtubers talk about Duct seal, which seems the easiest, but seemingly not available in Aus. it is frustrating me, so look forward to tapping into your wisdom.....

Silicone on plastic kayaks, sikaflex on composites. Selly's all-clear or marine goop. If you warm it first by placing it in a container of hot water it will flow out better and gve you less bubbles.

I've found Sikaflex 291 works quite well on polyethylene.
I first clean the surface with acetone, then rough up the surface of the hull with 60grit sandpaper.
If you use enough sealant in a large enough "dollop" push the transducer into it slowly while rocking gently, making sure it stays in contact with the sealant. Aggressive rocking will create air bubbles.
Had no issues whatsoever.