Bondo was a huge worry for me when I built my 300. I used to work in autobody shop and we were extremely careful to ensure that the bondo saw limited amounts of moisture. That said at the direction of the manufacturer I used bondo on all the knot holes in my plywoord. So long as there is no compromise of material it would be fine. unfortunately there has been a compromise. I wish you well and for your sake and all of ours don't give up now. This WILL be an awesome tank when you are done. Keep at it!!Yeah that's what pisses me off.. Nothing appears to be loose at all.
Pretty sure though now that the bondo is wet under the PA that is might be crumbling? If I remember right bondo doesn't do well in water.
Another huge design flaw perhaps.