Should not really need coating if its made and sealed well with waterproof PVA but you can put hot wax coating inside to seal any small holes. Thats probably the easiest.
I usually fill the feeder with water (in the garden) to see if it leaks. If not I use it as it is. The bees can easily walk up the inside of a wooden frame feeder to get back out again. If there is a leak I get some PVA wood glue and use a brush like Hedgerow Pete to get it into all the joins. Takes a day or so to dry.
With a plastic feeder you would not expect it to leak but bees find it hard to get out again when they have taken some of the sugar solution. Quite a lot of them die in the feeder. To avoid this before you use the feeder for the first time put some glue on the inside sides of the feeder and while it is wet sprinkle a little sand on the glue so the bees can get a grip. Allow to dry before use.
These plastic feeders are new and hopefully the manufacturers will soon realise the problem and make the insides a bit rougher.
First Th*rnes Rapid feeder I bought was as smooth as a babys bum in the central well where the bees climb up ..I took a piece of 180 grit abrasive to it and I've not had any problems with bees drowning or having any difficulty climbing up. Second feeder I bought seems to have been modified as there is a 'ridged' interior to the sides of the well ... no problems with this one either. Just make sure you check and if it's smooth sided do something about it.