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Originally Posted by Big fish
Sorry but another question. After grinding the inside of the outer skin I didn't grind it down evenly and there is some wavy spots. Will this uneven surface be a problem when bonding the core to it? Thanks again for all your help and I can't wait to see more of your progress.
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Most of the time when you do a laminate you always start with csm (matt) and that will help hold more resin to fill in the voided areas. If the voids are real bad you may want to add 2 layers of 10oz matt to give more thickness to fill them in.
There are two ways I usually do a transom to the original skin. The first is if I use Poly or vinyl-ester. I start by adding a layer of 10oz matt then a layer of 1708 over that then 2 layers of matt over that for the inside laminate between the skin and core. That will give you skin-matt-matt(on back of the 1708)-biaxel (top of 1708)-matt -matt. This gives you two layers of matt to fill the voids between the skin and the layer of biaxel. Then the two layers of matt on top of the biaxel will help fill the voids to the core material. This creates a good bond to the hard surfaces that you lay up on or to. This leaves the biaxel in the middle which is for strength mainly. The core gets soaked with thinned resin before it is installed and then I will add several layers of 1708 between the two 3/4" wood plys. Always put the matt side towards the bare wood. At this point I will clamp it inn place and wait for it to set up. Once dried I will add the fillets around the edges and then start laying up the outter layers of glass. Matt should be used always as a first layer up against old glass or wood to gain bond strength by holding more resin and filling voids. Most people like the 1708 because it has the matt glued or (stitched for epoxy) to the back of the biaxel fibers. You can lay it as one layer and it is faster maybe easier to do than two layers separate.
The second way I may install a core into the transom is with epoxy. I then use just a thickened epoxy resin with cabosil then trowel it directly onto the surface of the skin. The trowel I use will be notched with 1/4" teeth and I get it thick enough it will stay in place and not run down the skin. The thick resin should fill most voids and dips. Once this is applied I stick the core to the skin and let it bed down into the thickened resin. I try to get all the air out if possible and clamp it in place to set up. Dont squeeze the resin out from to much pressure. Try to keep as much in as possible but at the same time bed it well enough to get all the air out. The epoxy has a strong enough secondary bonding strength that it can be used with out matt and will bond the core in well. It will not provide much strength as that comes from the cloth. You need to make sure the original skin is plenty thick to do it this way. This is how I did the 74 20' sceptre. The skin was super thick already and I added more bixal to the inner section between the core wood plys and the front side of the core to give it plenty of strength. If I bond it in this way I will also will fully seal the wood core with two layers of matt first so that it will be completely seal from any possible water intrusion. I scuff it real well with 36 grit to give the surface some "tooth" for the epoxy to bite to. The thinned poly or ve will absorb into the wood better than the epoxy. I usually thin with styrene or you can use acetone. (The first method I described to you I just bond the wood in without enclosing the core first. It gets enclosed as it is laid up.) Once the epoxy sets up you can add all the fillets around the edges and then laminate the front side of the core to the boat.
There are many ways to do a transom but that is the two main ways I do them. Hope this helps, Mike