The cockpit locker lids on Live Magic were quite worn when I bought her 2 years ago. Since then thanks to them soaking up the rain they have worn quite quickly and have also been allowing water into the cockpit lockers. Thankfully this is only rain water but having to taste bilge water is not on my list of fun activities on a boat!
This year I have decided to replace the lids and will be painting them to match the rest of the deck. To begin, I bought a sheet of WBP plywood 18mm thick. This is the thickness of the existing lids and makes them flush with the cockpit seats. Although many would use marine ply for this application there is simply no need. The difference between WBP and marine ply is purely one of quality, with marine having no internal voids or filler. WBP is made of the same wood and uses the same glues but there may be some small voids and some filler used. Since I'm painting the lids anyway I'm happy to fill any remaining voids, and will be fully sealing the wood to keep moisture out. If I was planning on a varnish finish then marine ply would have been ncessary to make the lids look better.
Plywood sheets have an interesting pricing structure where a sheet is only very slightly more than half a sheet which is only slightly more than a quarter sheet, so I recommend buying a full sheet so you have sufficient spare material in case of mistakes, and also to allow for defects to be worked around, so you can choose the best piece for your lids. It's worth mentioning that a full sheet won't fit in a car so you may need to use a van, roof rack, or cut it in the shop or carpark to get it home. Alternatively you could get it delivered by the shop.
Next comes the cutting. Lay out the sheet on some saw horses and find the best parts. Place the old locker lids on top and draw around them, or measure the holes and mark them out. When drawing around old lids, you may need to mark the general dimensions and then fill in lines with a straight edge afterwards if like mine they are very worn. Use a table saw if available or circular saw if not. Don't use a jigsaw if you can help it as you'll struggle to get straight lines. It may be easier to cut rough rectangles around the shapes before the final cut so that you're working with smaller pieces of wood which are more controllable on the saw. At this stage, try the lids on the boat and mark up any hinges and locks for cutting. You may also want to write "top" or "front" for reference if you have odd shapes.
Once you've cut the shapes out you'll want to do some finishing. First and foremost, use a very sharp plane to straighten all edges. Because plywood is laid up with opposing grains for stability a blunt plain will tend to rip the end grains so make sure it is sharp before starting. Remember that a plane is to flatten the bumps and sandpaper is for smoothing to a finish. There is no point trying to sand something flat, even with an electric sander, you'll get bored long before you achieve your goal! I decided to cut the hinges into the lids to give a tighter fit. To do this you can either go the traditional chisel route, or use a router. I used a router, and clamped the lid to the side of my workbench to do this. Make sure they are very securely mounted before using a router. I first cut the ends square with a chisel to get a good finish, and then routed between these marks. I set a standard routing bit to a depth appropriate for the hinges and made the cuts. Next, I used a curved bit to round the front tops to match that of the boat and to provide a more resilient shape. A perfect square edge will wear very badly so try to avoid these anywhere on the lids, and always round off the corners a little with sand paper.
Once all hinges and locks have been cut in, start sanding. An electric sander is fine for this but you'll probably also need a flat block of wood and some hand paper for the fine detail work. Start off with 80 grit and work down to 120 or 160 for a fine finish, making sure to round off corners where appropriate.
To finish, I decided to apply two coats of exterior varnish to seal the wood, sanding with 160 grit between coats. After these, a final sanding and then the painted finish gets rolled on just as I did with the deck. The top side will have a non slip finish using non slip pearls. Don't try to save money and use sand here as it destroys expensive sailing gear. The pearls are only a couple of pounds even at a chandlers and since they are plastic will not wear out your oilies as quickly.
I'll post pictures later this week of the finished lids when they are done (and the new washboards!) and then hopefully some of them on the boat next weekend.