I've saved this for last because I want the reader to pay close attention to this section and to remember it.
Side note: Years ago, when I was working on a casino boat in the Florida Keys, the company owners hired an outside consulting firm to come to the boat and teach all the supervisors tricks for training new employees. In one demonstration, the consultant read a list of twenty random items; pencil, elephant, razor, hat,...
We had to wait for sixty seconds, then right down as many as we could remember. Most people could remember between 6 and 10 of the items. Interestingly, every single one of us remembered two specific items. The reason we all remembered these items is because they were the first and the last items mentioned.
When people are given too much information at once, they can't digest it all so they remember what stands out to them. For most people, this is the beginning, the end, and something interesting in the middle.
This trick has always stuck with me and I'm using it here by in the hope that it will stay in the mind of the reader.
Pourable cores are good but not as good (or as easy) as most people believe.
The concept is an easy sell to a Do-It-Yourselfer. Dig out the old core leaving the fiberglass skins in place, then mix up the pourable core and fill the void. In a perfect world, the core is perfectly bonded to the skins and the repair is a permanent solution.
Like most things that promise too much, it can't always deliver.
Before I continue, I want to stress that, prepped and used properly, pourable cores really can deliver what they promise. It's the prep they gloss over in the manufacturer's literature.
Let's start with what it is. Pourable cores are basically straight resin thickened to a point where it will fill gaps like a fairing compound, but not so thick that it can't be poured. It can be structurally improved by adding bits of chopped fiberglass strands, and its resiliency can be improved by adding elastic polymer fillers to the mix.
Up to this point in the book, MEKP has been listed as the only hardener for use with polyester and vinylester resins, and proprietary hardeners listed as the only option for epoxy resins. Pourable transom cores use a different kind of hardener, Benzoyle Peroxide (BPO) because it generates less heat in the curing process (which also slows down the curing process).
If you tried to make your own pourable transom using polyester, vinylester, or epoxy with the hardeners typically used, you would probably burn your boat down to the ground. The volume of resin would generate so much exothermic heat that it would almost certainly catch fire.
I'm telling you this because it's very likely someone will think, “If it's just resin, I can do it myself.”
BPO is not a suitable alternative to other catalysts mentioned in this book and should not be thought of that way. The only time it should be used is when it is expressly intended by the manufacturer. In this book, this will be the only mention of BPO as a catalyst.
Here are my problems with pourable transom cores:
First, like prepping any fiberglass area for repair, the surface must be contaminant free. This is done by grinding down to bare glass, vacuuming out the dust, and cleaning the fresh surface with either acetone or denatured alcohol. If the surface hasn't been prepared in this manner, the bond will be questionable at best.
The problem is, without removing either the inner or outer skin, the inside surfaces cannot be adequately prepared for the new core. The tools don't yet exist for abrading, cleaning, and solvent wiping an area that is five feet wide, two feet deep, with only an inch and a half of clearance.
Trying to avoid the removal of a fiberglass skin will result in poorly bonded sandwich construction. When fiberglass is not bonded to the core, the result is a transom that is only as strong as the thickness of a single skin. This is an impending transom failure.
On the other hand, if one of the skins is removed, providing access to the inside surface of both skins, then they can be adequately prepared and the removed skin glassed back in place. When done in this manner, a pourable transom is the best, strongest, and easiest for the at-home boat restorer.