Tar thick on the outside. If they are wood floors keep them up out of the ground on a good base of gravel and the weeds/grass cut around them. Also found the steel flashing you put on the floor instead of tar works great. Have 3 bins with and 3 without. The 3 without needed new floors and the 3 with look brand new still.
When we redid some floors a few years ago we cut up some old tractor tire tubes into strips. We put these strips between the wood floor and the bin ring. We made the strips wide enough so that it would get pinched between the bin and floor but also hang over the edge covering it and the side of the floor completely to the ground. Then we put a layer of tar over top the seam on the outside. Not a leak yet.
We also put all our flat bottom bins up on railway ties to keep them out of the ground.
CMC sells a grain bin sealant. It is a one part latex, much like very thick paint. There is a fairly open weave cloth you embed in the sealant, and then apply another coat over it. I did this about 5 years ago, and the sealant still looks good. The advantage of their sealant over tar is the sealant remains flexible in the cold.
In my case, the latex was covering tar that got old, and then shrunk and cracked.
Some good things to look into. I've tried several different types of tar with mixed results, and certainly cold weather seems to be the bigger problem with tar. I wish we would've put something better between the bin and the concrete when they were built but the builder was adamant that his product was the best ever, I'm thinking he was a sponge salesman in a past life as his seal sure acted like one but too late now. I'm thinking a latex based product with some sort of fiber reinforcement would be a good next choice but I'm sure not looking forward to scraping all the old tar off.