The companionway hatch cover on my Victoria 18 is cracked and needs repair. I believe the construction of the hatch, like the deck, is balsa-cored fiberglass. I'm looking for advice on material and technique for a strong repair. What weight of glass cloth? Do I replace the core? Etc.
Cracked - is that surface issue only? Check before you do too much.
Companionway hatches are seldom balsa-cored, usually in "pure" grp. Check.
If there is balsa, you have to check if water has penetrated or not. If the crack(s) are deep, then probably the balsa is to be renewed - or a design change.
Companionway hatches are often quite exposed as they are in the middle of everything. At the same time, forces on these are often limited, resulting in damages being on surface rather than deep.
I propose to start with a real clean-up. Start to get of dust and grease with detergent and water, continue with either a solvent as aceton (strong) or some light rubbing.
In any case you have to investigate if there is balsa or not. If you strongly belive in balsa, then drill a small hole, preferably from the underside. It doesn't have to go throu to the up-side. Then you will get out some balsa (if there is any) and/or some grp. Carefully inspect. Wet balsa means you should remove it. GRP should not be wet, but if it is, then there are some different options.
If balsa is to be replaced then it is probably best to remove the grp layer from the underside. Best is to do this in-door, hatch removed. Easy to work then.
Remove balsa, put a new layer, and seal with some layers of either usual fiber + polyesther or use epoxi.
Then have a look on the upper side. Cracks shuod be repaired, often sufficient to use gelcoat. If upper side has considerable cracks, then grind of gelcoat layer, clean with aceton and add some layers of grp. īTop with gelcoat/topcoat (a lot of work to get even) or paint (simple, consider to make it intentional different then it was wrt pattern and color).
However, I suspect your hatch is without balsa, but has many cracks, some are deep. In that case, remove hatch, have it in-door to dry. Decide if you want to repair
a) with some layers of grp
b) cosmetic with gelcoat / topcoat
Cracks should be filled in the cases of (b) and (c).
There are a lot info on internet on how to work with grp - google, look up and read before you start.