Deckhead repair

[Tim Stewart]

Thanks Brian for your quick response.
Sorry for not mentioning what type of boat I had. I was in a bit of a rush when I was writing. It is an Achilles 24, built locally in South Wales, UK. in the 70s. It is fibreglass, plywood, fibreglass sandwich construction. I was going to attack the job from underneath so as not to disturb the visable topside finish. The underneath is rough finished fibreglass covered by the deckhead lining, so it can be covered up again when I've finished.
The mast has already been removed.
I was planning to cut out the bottom layer of fibreglass very carefully with an disc cutter. Then pick out the 'mush' plywood. I was then going to place a fresh piece of marine ply or a solid piece of plastic sheeting in position,jacked up in place, and glassed over.
Do you think this will do the job.
Could you please let me know why you prefer attacking a job like this from the top. As I would find it harder to make the top look as good as it was prior to the start of the job.
Thanks again.
Tim Stewart.