Marv, in answer to your question, some prefer to pre-apply a coat of resin to the bare plywood.
Your call!
IMO, the plywood core will be your best scenario.
Marine plywood is not necessary in this size hull, but is certainly a better choice over that of CDX (of which many manufacturers used).
This essentially becomes a "Box Beam" when done! IOW, a core separated by two webs.... (the original hull and the final interior layer of heavy Roving).
NOTE: Fir Wood is one of the better plywoods to use. It holds up well against moisture, yet offers good strength.
Plywoods to avoid (IMO) are; Birch, Maple, Oak, Pine, Cedar, Hemloc, and for reasons of both moisture and strength.
Oak, for example, is very strong, but it lousy against moisture.
Cedar, for example, will fair well against moisture, but is not as strong for this application.
Fir simply seems to be a better all-round choice.
Here's my take on the procedure:
Determine which product was used for the original build. Polyester or Epoxy.
Older hulls were often Polyester.
Since this hull requires a new core, it is likely an older hull...yes/no?
Have all of your materials ready: Matting, Roving, resin, appropriate thinners for clean up, brushes, all screws/fasteners, etc.
Take the old transom core material out and down to the original hull glass.
Cut the core into small sections, and it may be easier to remove.
In the past, I've used a router and straight flute bit (set to depth) and a pneumatic chisel for this.
Of course you'll want a disc sander, resperator, baby powder and glooves as well..... and perhaps a fan to give yourself good air to breathe.
Cut and "Dry Fit" your first piece of plywood.
It may require taking In/Out several times.... but make sure that you have a good fit.
If you must section it, do so strategically.
Secure this (these) to the hull by any screw means that you can find...... transom unit mounting holes, TT plane holes, swim platform bracket holes, etc.... even if you must add several (they can be filled in later on).
NOTE: Run these layers right on through the transom cut-out area.... the cut is made after all work is completed.
Be prepared to somehow fasten a straight-edge "backer" to the outer section of the transom (with the OEM core removed, this area may flex and no longer be straight/flat)
Now cut and fit your second core piece.
If it too must be sectioned, be sure to stagger your cut from that of the first core plywood piece (if sectioned).
Again, it may require taking In/Out several times.... but make sure that you have a good fit.
Now secure this from the engine side, using as many fasteners as required.
All pieces are now dry fit into position.
Examine them.... look for any areas that may still need attention.
If reference marks are needed, be sure to make some.
Now pull everything back apart, labeling all pieces and noting where all fasteners go.
The next step will be done within time constraints.... so the dry fitting will now prove to have benefited you.
The chopped glass strands may not be an option for you. This is typically a "Chopper Gun" task.
I'd recommend heavy resin-wet matting for the first application against the hull's original glass transom.
This is fairly easy to wet and lay up.
This layer of matting forms the bond between the two.
Using all previously arranged fastener holes......, install your first plywood core (also resin-wet) against the resin-wet matting applied section.
NOTE: This is also when the "backer" may be applied to ensure that the first core application holds the immediate area straight/flat for the later installation of the out drive transom unit.
This must be straight and flat..... once that first layer's resin cures, you are locked in!
Chink all areas around the perimeter with sheaded resin-wet matting, as to close up all voids.
Now you're ready for the second core material application.
No need to wait for first layer to cure.
Again, apply resin-wet matting to the now inside of the first core layer.
Wet the mating side of the second piece as well.
Fit and secure the second core screwing it to the first core using a screw pattern that ensures complete mating.
Again, chink all perimeter areas.
Now you're ready for the last layer of heavy resin-wet matting, followed by a layer of heavy resin-wet roving.
This will be your last and finish layer.
Most resins will require a working temperature range, and will suggest a minimum working temperature.
The temperature will also be part of the equation when mixing the catalyst.
Give yourself working time when you choose the ratio. Working temperature can be a big factor in this.
Have fun!
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