I'll be using regular plastic buckets to make some concrete planters, something like this. My goal is to reuse the buckets.
The problem is that she sprayed each bucket (generously) with canola oil and non-stick oil, but the dry concrete stuck really well to the bucket. She ended up having to cut both buckets to remove the concrete planter.
Is there anything I can spray the bucket with so that the plastic doesn't stick to the concrete?
A simple solution I used once was a layer of cling film... But you have to get it smooth... Any imperfections may show.
I found using car wax was a great way to get concrete to release from plastic buckets, I wiped it in the buckets and made the rounds with the patterns my wife asked for. A couple of days later I turned the buckets over and most of the pieces slid right out, the couple that had to be thumped got another coat of wax, this held up for 3 pours and that was all my wife needed.
We can't make specific product recommendations here, but you need to do a search for what's generically called "concrete release agent". It's usually a silicone based spray, but there are various types.
Liquid sprays that don't dry wouldn't survive the initial pour. Using solid vegetable shortening on the sides should make it stay there, otherwise you could use a liquid or solid wax that dries to a film. Something that they use on cars maybe?
Regular 5 gallon buckets have mostly parallel sides, so any bulging will make the concrete wider than the top and not want to come out. You might want to check out getting different buckets too.
The guys that taught me volume concrete work just used diesel and water, or a light machine oil with a pump sprayer. I certainly agree that the bucket will expand into a non-cylinder when you fill it, and sticking is not going to be the problem. Unless you need to have the inside form exactly matching the bucket, maybe you should remove the inner bucket before it cures. There will be slumping, but you may not care. You probably should be putting a drain hole in the bottom anyway, and that would be a good opportunity to ram through a small rod to make a hole.
