So, the job is to cut out an opening in your 3/4 inch plywood cored fiberglass deck. What is the single "right" tool to make the cut? Answer. There is none. The fiberglass will dull any toothed cutting blade in seconds (i.e. jigsaw or sawzall blade). Carbide grit blades work great on fiberglass and last a long time, but they wont cut plywood. In fact, carbide grit blades have a greater chance of setting the plywood on fire, due to heat from friction, before making any significant cut (don't ask me how I know).
So, what to do? Answer: use the "wrong" tool, and expect to ruin a number of blades in the process. But there is a way to go about it to minimize the tool cost:
- Go to the hardware store and stock up on toothed and carbide grit jigsaw and sawzall blades.
- Rough cut the opening just short of the desired opening size with a jigsaw and/or sawzall with a toothed blade. Expect to change in new blades a couple times. This gets most of the material out of the way and close to the edge of the desired opening where you want to spend time on detail.
- With a router and sharp rabbeting bit, carefully route out the plywood core. The router will fly through this easily. This gets the plywood out of the picture.
- Now, switch your jigsaw/sawzall to carbide grit blades. Cut the fiberglass even closer to the desired opening size as much as possible, without cutting beyond. It helps to have a plywood template cutout jig if possible.
- Repeat steps 3 and 4 as necessary to get closer to the desired opening size.
- Finally, trim the edges of the de-cored fiberglass with a router and trim bearing bit. Having a cutout jig here is a must unless you are quite good and freehand (I'm not). The blade on the router bit will be dulled here, so have a couple good quality bits on hand. The closer you can get with steps 3 and 4, the easier this step will be.
- Do the final rabbet of the plywood core to prepare for epoxy/resin fill of the edges.