Because my hatches were very thin after removing the rot and grinding
I used 2 layers of 1.5oz mat wet out real good with polyester resin then placed composite down and weighted the dry side down with builders blocks, batteries and such. Drilled holes in the composite about every 4" beforehand to allow air to escape. Let it kick, sanded the resin around the edges and that which poked up thru the holes and glassed the backside. Gorilla strong.
I am not fluent with epoxy. So don't use my comment as "the word" but
If you are using epoxy with thickening agents, I would think you would still want it thin enough to put on with a roller and let it flow out flat. If it is thick enough to trowel on you would have more hills and valleys creating voids causing you to put more force on the wood coring to get the air out. If on the other hand you were doing a vertical transom and were using transom clamps I would say go ahead and make it thick, trowel it on, clamp the coring down good and tight until the paste is oozing out from behind the coring and wipe off the excess to save time later.
Someone will chime in and rip me a new one but this is my logic for what its worth.
See BenFishin already did. Mayo or Miracle Whip? My thinking was that air bubbles move more easily thru unthickened epoxy than mayo or peanut butter