Hi Daimo,
Firstly, some of these old boats were just under built and may have had a little bit of a bouncy deck from day one. I know the Hoods were notorious for this.
Not a bad idea to investigate before cutting the outside skin.
Sometimes looking from underneath is a good first step as the may be no gel (flow) coat. The inside skin is only a couple of millimetres thick. Bad rotten core can (should) be black and sometimes can be seen. Grab a torch and have a good look.
Also have a bit of a tap around (20c/50c coin is a good tool) from underneath and work out where sounds soft (dull). Outline the area with a pencil.
When the vent is out check the core to see how wet it is if it wasn't sealed properly.
Next step can be a couple of drill samples from underneath with a 6/8mm. Should be able to see if the core is wet or damp. Sometimes
water actually drips out! If it is hard to tell, the smell will give it away. Rotten core stinks!
I agree with roverhi with the drill/epoxy squeeze in the holes. Once drilled the hole, a de-coring tool made from a 4mm Allen Key with the tip ground off in a slow drill is useful. Removes core but allows more glue to go in.
Page 4 on
https://stevedmarineconsulting.com/w...s-May-2014.pdf
However this can be done from the underside using thickened epoxy in a syringe and a spot of tape to keep it in. This method is cutting corners but is effective and quick.
If you are going to bite the bullet, I'd go from underneath if possible. Deck
repairs can be VERY hard to make seamless. It is possible to achieve good bonds with proper preparation when doing a dry fit. And yes it will be messy, but thats half the fun. Again preparation for mess is key.
Private Message me for repairers in
Sydney if you want mate, was a shipwright there for 14 years until recently.
Good luck!
Adam