I like printing PLA vases, but they all leak water, making them unsuitable for flower vases. Someone said spray with polyurathane, is this a good method? any suggestings for type or brand?
Have you tried increasing your walls to 8?
Haven’t done many vases but I pour FlexSeal into the vase, roll it around to coat the inside well, then let it drain out. So far that has worked.
Here’s what I used.
Here’s an example of the product I made with it. This was the largest and heaviest object I printed in my printer. Approximately 240mm tall.
So far it hasn’t leaked and my wife has not complained and believe me, if she could find a reason to complain… 
I should add that this triple-thick formula requires only one heavy coat and let it cure for 72 hours before filling the container with soil and water.
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This is one of my latest prints related to this topic. It holds (flawlessly, I might add), water, beer or wine. I kept it full with water for a bit more than 24 hours. No leaks, and that without using any seal or spray… perhaps i was lucky.
For projects like this do you increase the number of walls or is that irrelevant outside of strength? In short, is the Rust-Oleum doing the work of holding water or a combination of that and more plastic in your experience? Thanks a bunch for the recommendation by the way. I have been wanting to print some vases for my wife, but I needed them to actually hold water 
Increasing walls will definitely have a positive effect, especially when using sparse infill at low percentages or doing any kind of post-process cleanup with let’s say sanding. However, there is a limit. For example, if you’re getting see-through impressions of the underlying infill structure, such as a honeycomb pattern showing through the surfaces, then switching the wall order from the default inner/outer to outer/inner and increasing the wall thickness will force the exterior walls to firm up first, giving a smoother appearance and aid in water tightness. But there is a limit. I found that anything beyond 3 or 4 walls doesn’t really improve it. The default is 2. Note that this can dramatically increase print time.
However, as you already surmised, the spray paint is really doing all the work and I have to confess that the triple thick spray paints really are the lazy man’s best friend that I unabashedly resort too when I am too lazy to smooth a print. You don’t have to use Rust-oleum either. Any polyurethane triple thick will work.
https://www.amazon.com/s?k=triple+thick+glaze+polyurethane
I’ve had luck brushing it on too. It just sort of smooths out on its own. I did find that a heat lamp helps accelerate the curing process but be careful. Too much heat and you get this affect. Not one of my finer moments. 
Fandamntastic! I learned more in that post than the last twenty I have read trying to solve a different problem. It only took me 48 years to learn there was such a thing as triple thick spray paint. Always a good day when you learn something new…especially at my age.
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Very cool!
I also printed an easy one out and it worked just fine. But I want to try also to print it with PET-G