I searched the forum and did not see any posts regarding waterproofing the inside portion of the feeder. I am going to build a top feeder for my hive similar to the one sold at Brushy mountain (http://www.brushymountainbeefarm.com/8-Frame-Hive-Top-Feeder-w_Floats/productinfo/262/) . I see several non toxic waterproofing sealants to purchase but was curious if anyone has already done this and if they had any recommendations.
I would use only 100% pure silicon as a sealant. Anything rated for kitchen or bathroom will have fungicides in them to inhibit mildew and may be a source of contamination to the bees so I would not risk it.
I coated the outside only with Tung Oil but melt n swish bees wax around the inside to seal the seems n inside joints. 
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. Seems to work okay n very natural n free if you save all your scrap bees wax. I had to buy some this first round.
That’s what I did,
Gerald
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John, our weather isn’t HOT here near Seattle … Our Springs n Autumn when we use them here are cool …, so I’m not dealing with extreme heats here in the Foothills of a Puget Sound.
Where are you at. Is it Hot during feeder usage there. I am storing upside down in the shade. We’ll see how summer treat them but doubt our temps will melt the wax here.
Hope this help you bro. It works for us here. Many locals use this wax sealant here. It wasn’t my oridinal idea. So must work okay locally.
Gerald.
July and August can be in the 90’s with some day’s close to 100. Although I hopefully won’t be feeding in those months. I guess all I can do is give it a shot.
If you are that warm. Maybe store upright n it can only melt inside. Just food for thought. Your chilly winter should make up for the summer heat. I have friends that live in the Bitterroot n some in Whitehall n Sherdan. They were I Twin Bridges for awhile.
Great idea, thank you for sharing, Jerry! 
Gloss paint …
Dee,
I’d guess Glossy paint works too. I’ve just tried to use something I can touch up n keep the paint out of the inside. Even the feeder the bees seem to have propolis some already. I see many commercial feeders are made of plastic.
Gerald
Call me cheap or simplistic, but I use a 1$ polypropylene chicken feeder on top of the crown board and a double. Put some stones in the chicken feeder to prevent the bees from drowning.
Simple: If you need the double for honey, then you should not be feeding anyway and if you don’t have honey, the double is surplus. But, we have warm weather, so a void double is no problem.