I'm replacing the transom in an old Jon boat I bought. I'm planning to use 2 pieces of 3/4" cdx plywood cut to shape, glued together then laminated. My question is how do I laminate it? The guy in home depot said to buy fiberglass resin and cover it. No matting or anything just cover it in resin. Is this correct?

Send a PM to Fishing Rhino. He da man when it comes to dis stuff. 

The resin will waterproof it.  The matting for fiberglass is to add strength, you don't need strength you need waterproofing.

Polyester fiberglass resin and plywood is not a good combination.  In a couple of years the resin will start to crack, allowing moisture to get to the wood.  The process accelerates from there.  Epoxy is the way to go.  It will make a stronger bond between the two layers of plywood, and will do a much better job of sealing the wood from the elements. 

 

However, if you are planning on keeping the boat for a few years, I'd rethink the CDX, which if I'm not mistaken is used for sheathing, subflooring, and roofing for houses.  It is about the cheapest grade of plywood.  Structurally, it is strong enough, but it can be loaded with voids in the outer and inner layers of plywood.  The CDX indicates that one side is grade C, the other grade D and the X indicates it is bonded with exterior glue.  It generally consists of fewer, but thicker plies.  This makes it cheaper to produce, but less suited for what you are doing.

 

Try to find a PTS plywood.  It will have some interior voids, but the surfaces will be Plugged, Touched (filled) and sanded.  Will make for a better bond, and tighter surfaces.  I forget the grades but think it is either ACPTS or BCPTS.  Even though epoxy should do the job, A good coat of exterior grade paint will protect the epoxy from the UV rays of the sun.  No need to spring for the expensive marine grade paints. 

 

One other thing about polyester resin.  It is not waterproof.  It will absorb moisture, epoxy and vinylester resins will not absorb nearly as much water. 

 

 

  Epoxy Resins vs Polyester Resins

To: c22@catalinadirect.com                                         Subject: Re: hull fairing                                          From: "Kevin" kls@ipass.net                                        Date: Sun, 12 Sep 1999 00:28:23 -0400                              Just a few facts that I know concerning the comparison of polyesterresins to epoxy:                                                                                                                      Polyester is not an epoxy.  It is a complete resin system that willslowly polymerize when it's solvent is evaporated - though this maytake months. That is why MEKP is added.  It is a catalyst - not a  component of the resin system - that speeds up polymerization, or  curing.                                                                                                                               Epoxy is a true two-part resin system in that polymerization will  not occur - over any length of time - unless the two components aremixed.                                                                                                                                Why epoxy is a better barrier coat than polyester and why epoxy    doesn't blister...                                                                                                                    Epoxy is 100% solids (high quality epoxy, anyway, such as WEST,    System 3, and others) meaning that there are no solvents - the lackof solvents is why epoxy resin and hardener do not have much of an odor, though some odor.                                            The weight of cured epoxy will be the same as when it was liquid.  Polyester resins have allot of a solvent called styrene and this   styrene evaporates as the polyester cures - this is why polyester  resin smells really bad.  This evaporation leads to porosity withinthe cured res.  The porosity is how water eventually permeates gel coat and causes blisters. All plastics absorb some moisture,       chemically speaking, but polyester absorbs way, way more water thanepoxy.  This is why epoxy is an excellent barrier coat.                                                                               Epoxy is an adhesive. Polyester, though it has adhesive qualities, is not as good for bonding as epoxy... 

http://members.tripod.com/c22_fleet_6/epoxy-vs-polyester.html

 

 

Thanks for all the info. I'll use epoxy instead and try to get some better wood. I appreciate it.