Definitely cut the hatches and finish sand them and the lips the hatches will rest on. All those surfaces, especially the exposed "end grain" of the cut plywood, are very vulnerable to water damage.
One other item to consider: the holes in the deck needed to mount hardware (pad eyes, etc.) are also vulnerable. Many builders pre-install hardware, remove it, and seal all those holes with their exposed endgrain prior to final sanding. I usually drill the holes slightly oversize, pre-install, remove, and saturate the newly exposed wood with resin, and then drill the holes to finished size. No need to spply varnish to the holes, because they will never see any UV.
I think that is what I was thinking! I plan on cutting out the hatches, using epoxy to seal the edges and then varnish the deck (and hatches) for a final finish. Was thinking of a final epoxy coat before the varnish, but don;t think I need it since the varnish should provide a good finish after the wet sanding. That sound reasonable? Thanks for the notes.
I would not skip the epoxy. It is stronger and more resistant to mechanical abrasion. With it absorbed into the wood, any minor scratches will not penetrate the wood. Varnish is not very strong, comparatively, but it produces an attractive shine, and, most importantly, protects epoxy from UV damage. Over time, the varnish will degrade mechanically, and the hull will need another varnish job ... years off, maybe 20 years. But if water gets under the varnish, the wood can rot, and any repairs are a hellish task. Walk down the floats in any marina and look carefully at the wood trim on the sailboats. Lots of damage.
Be sure to leave 'allowance' for the epoxy and varnish when you are sizing the hatch lids. On the first boat I built with wood hatches, I was very proud of the close fit and great job I did sawing out the lid from the deck. After epoxy and varnish, the lids wouldn't fit, so I had to sand the perimeter of the lids smaller and re-do the epoxy/varnish!
Thanks, I have a couple of coats of epoxy on over the fiberglass, guess I was not too clear on my question. I was wondering about putting a coat of varnish on over the epoxy before cutting the hatches. Seeing John's message makes me think think that might be a good idea to avoid a tight fit problem. thanks for the replies!
Tobin, you can varnish the hull now if you want. But you will still need to seal the cut edges, and the new plywood installed underdecks as well, after installing the hatches.
Varnish applied now does not prevent a tight fit later. The width of the saw kerf controls the width of the gap. However, even with a saber saw kerf, the added epoxy and varnish applied to the cut edges may build up the edges enough that the hatches will jam. After cutting the hatch openings, it is wise to clean them up with a few passes of 80 grit sandpaper before applying resin and varnish, and afterwards. When you have cut the hatches, you will see what we mean.
Dave, I was going to ask how you can drill holes oversized, then drill them to finished (smaller) size. Trying to image a drill that would make the hole smaller
But now I understand that you fill the oversized holes with epoxy - coating the inside edges - then drill out the epoxy with holes of correct size, leaving some epoxy between mounting hardware and wood edges. Is that correct understanding or do you have a magic, "make the hole smaller" drill bit.