First, I think the long-term purpose of this cage is important for us to understand. You mentioned that this is a temporary cage.
1) How long do you forsee keeping a monitor in the cage?
2) What will it house next?
The above information will help me a lot when it comes to making a recommendation. I'm going to deal with the rest of your questions in reverse.
I have already applied one coat of sealer (Helmsman I think was the brand urathane of some sort). But I only applied it to one side so if need be that could be the outside and I could still seal the inside with whatever is best.
I do not like the use of Helmsman Spar Urethane in reptile enclosures. First, it is designed to be highly flexible and resistant to UV light, both properties which will mean a LESS water resistant product comapred to regular polyurethane. Also, due to some unique properties of the product you used it can take years to offgass completely. This can be made worse if it's use on the interior of your cabinet, like in your reptile cage.
Lastly, I don't know how well polyester resins will stick to it if you do decide to go that route. Different clear coats don't always like to "work" together. The last thing you want is to apply a coat of polyester resin and find it peeling off.
FRP? I have already purchased the plywood which was not inexpensive.
I can understand your concern, especially if this is only a temporary cage. Are you really wanting the clear coat look on the inside? But FRP is hard to beat for a monitor, that's for sure. Not the most attractive stuff. I hate the way it looks to be honest.
Also, you'll waste a lot of FRP with the dimenions you listed. I'm sure you already noticed that with your plywood. You could have built a cage with a very similar volume and probably used one or two fewer sheets of ply. But maybe you had a reason.
Tell me more of this polyester resin. Fumes? Dangerous? Curing time?
There are different types of polyester resins and different types and ratios of activators so the above are hard to answer. I would definately plan on the use of a respirator at a minimum.
They are completely safe once cured, although that can take a while. One issue with polyester resins is that they don't lend themselves well to repair. It is hard to get another coat to stick after the product is fully cured. That is why multiple coats are applied on a certain schedule - it allows the coats to "burn in" to the previous layer. Wait to long and subsequent coats don't adhere well.
I want something durable but I also want to be able to tear this thing down and put it back together in a few months. Clear is not a necessity only a preference.
Then you would seal the individual panels before putting the cage together. If you sealed the already-assembled-cage the sealer would act as an adhesive on the joints and you'd be stuck with things for a while.
For something with color, I'd vote for FRP.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
1.2 Gonyosoma oxycephala - Jave local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)