I am a bird bath junkie. I have nine birdbaths out in my yard and will buy another one for an area I just spotted that could easily accommodate one. Don't know how I missed that available spot! I know, nine birdbaths is a lot but this is an agricultural area and there are many chemicals and feces from other species out there ending up in our ground waters and I figured water from my tap would be preferable to polluted ground waters that birds frequently drink. Needless to say, we get a lot of birds flying in to bathe and drink so at this point... what's one more birdbath to clean.
I have one very old birdbath here that is probably at least 70-80 years old or so. It is heavily pitted from exposure to the elements. It weighs at least 300 lbs. We bought it at an auction and it had to be delivered to the site and set in position so we can't very well move it in and out of the garage over winter to protect it from the elements. It's one piece construction as is common in older birdbaths so I can't even flip the basin over to stop it from holding water that freezes. It had a few cracks in it that I repaired using a tube type product purchased from an aquarium supply store that was non-toxic. The repair is holding however there is a new crack. Supposedly if we seal our birdbaths it reduces cracks.
My other birdbaths are all two piece construction and I can haul in the basins over winter leaving the base where I placed it or flip over the tops so water can't collect and freeze. One birdbath I leave with a de-icing wand in it over winter so the birds have at least one ice-free source of fresh water and that birdbath has taken a beating. I'll probably have to replace it next year.
Barring the above, there are a lot of avian diseases floating around out there. When we offer birdbaths to our feathered friends, we are encouraging them to congregate unnaturally which places them at greater risk of contracting an infectious disease. One of the reasons why it's recommended that birdbaths be cleaned thoroughly and disinfected every 2-3 days before adding fresh water. I've always been really conscientious about scrubbing my bird feeders and birdbaths once I was taught the "whys" of disinfecting them but admittedly only do so once a week on Saturday or Sunday. After I clean my bird feeders, I dip them in a 5 gallon bucket of 10% bleach and let them soak for a few minutes. The birdbaths aren't so easy to clean well even with a good brush, particularly that older birdbath that is pitted. After I scrub them all, I use the left over bleach water from the 5 gallon bucket to disinfect them then hose them to rinse them well before re-filling them. The real reason why I want my birdbaths sealed is to create surfaces that are easier to clean for me because the sealant will fill in all those pits. I don't keep up with all the avian diseases out there but they're there and pitted birdbath basins can be an ideal surface to harbor ickies. This is also the reason why I clean and dip all of my wooden nest boxes at the end of the season and why I am slowly but surely replacing wooden bird feeders with birdfeeders made out of non-porous materials. Non porous surfaces are faster and easier to clean for me.
As far as sealing the insides of my concrete planters, similar deal. They're too big to haul in and out of the elements and I'm told that sealing them extends their life. I've had quite a few crack on me over the years because we leave them outside year round and they are rather expensive. I recently asked a man who sold concrete lawn decorations what I could do about this. He stated I needed to add rocks to the bottoms to keep the drain holes free and that the planters had to be elevated off the ground a little bit to ensure free drainage and that I should have been sealing off all of the interiors. He said I should have my husband use a concrete drill to create drainage holes in any planters that didn't already have them and that it might be a good idea to add a few more drainage holes to large planters. He stated this would triple the lifespan of our concrete planters left outside year round. Two years ago I lost several planters. We had whacky weather and it rained for several days straight then the temps dropped and the water that was in my planters froze solid. They are above ground and tend to freeze faster than recessed planters. They literally split when the water expanded as it was freezing. Once I made the connection that I was losing planters to water freezing and expanding in them because of poor or blocked drainage, I realized what he said made a lot of sense. One problem, I can't use the cinder block sealer he recommended (highly toxic) because I've got chipmunks and squirrels rooting around in my planters.