Because of my allergic reaction to epoxy I experimented a fair amount with polyester resin. I am certainly no authority but this is what I found.
When trying to use it just to coat wood it seemed to cure very slowly and often not completely, making it difficult to sand without clogging the sand paper. I can't explain this. Simply adding a layer of fiberglass cloth seemed to correct this and it would cure and be very sandable. I read an opinion somewhere that the wood tends to absorb part of the chemical mix that needed to make it cure leaving the remainder on the surface. It was suggested to first apply a resin mixture thinned with styrene and allow it to penetrate and cure. Then coat and apply cloth. It was suggested if this was not done the resin under the cloth and penetrating into the wood would never really cure 100%. I tried that too and had limited success.
I will say I repaired the back door of my shop using polyester. It is a solid core wood door and has a direct west exposure. Gets a lot of sun and rain. Over the years the sun had caused the exterior laminated skin to split and peel away in some fairly good size hunks. I cleaned off all the loose material and sanded away most of the latex paint that still remained. I then used a bunch of scrap pieces of different weight glass cloth and polyester resin to cover the damaged area - about 3' x 3'. I honestly didn't do a very good job of prep or applying the glass because I had little hope the repair would be successful. That was 7 years ago and it still is holding up great. It looks crappy because of my inferior application - you can see the weave of the cloth and edges all over the place. If I thought it would have lasted I would have done a nicer job!
As to polyester over epoxy - I have read both it won't work and it will work! After reading everything I could find on doing this I decided if the area in question was above the wate line AND properly prepped it should work. I did a repair on the bow deck of my Cabin Skiff which is covered with epoxy and fiberglass. It was about a foot in diameter. I sanded off the paint and then used 60 grit sandpaper to roughen the surface. I then used polyester resin and glass cloth to cover the area. It cured well and I sanded and feathered the edges with no problem. I then repainted the entire deck. That was also 7 years ago and it is still perfect.
But two things need to be considered. One - the repair was above the waterline. Two - the boat is stored under roof and not exposed to sun 24/7. Personally, because of my door repair, I don't feel the sun would be a big problem but my thinking is below the waterline could be problematic.
I built a wood rudder for the sailing rig on an 8' pram. I covered it with polyester and fiberglass cloth. But I was too lazy to properly do the edges of the white oak rudder and just feathered the edges of the glass leaving the edges just bare wood. I then sanded and painted the whole thing. I knew this was wrong but really didn't plan to use the rudder much and thought it would get by. Well, the third time out I looked down and half of the cloth was flapping loose. I removed the rudder and proceeded to simply peel all the cloth off the wood with great ease. Obviously, the bare edges had allowed water to work under the cloth but I never dreamed it would come off so easy. What if this was a hull that got a scratch through the cloth below the waterline. My thought is the area could quickly start to de-laminate. This would NOT happen with epoxy.
SO, for my use I decided polyester resin is of minimal use and even then only above the waterline. I will admit on my True Grit I did use it and glass cloth to cover the anchor pulpit and so far it has worked fine. But if it fails I can easily remove it and make the repair. I honestly don't think I would want to commit to its use in large critical areas. The saving is just too small and the risk too high.
But again just my opinion. I know this does not directly address the question about using polyester to encapsulate the interior but thought it might still help understand the weakness of polyester.
Not to mention the stuff really STINKS!
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