What would you recommend for a spray on rust protector for storing an engine block? Thank you.

Trying to protect the whole block from rusting, or just the machined surfaces? Thick wheel bearing grease on the bores, or paint them with cheap paint and wipe it off later is good for storing core parts. Eastwood Anti-Rust is great too, but $$$ to buy it and ship it. It's like spray on wax coating.

Just paint it with a cheap spray paint and then in the future have it hot tanked to remove the paint.

Freshly machined or right out of a car? If freshly machined, heavy grease on the machined surfaces, Glyptol on the inner cast surfaces and the paint you intend to use on the exterior.

This is what my machine shop says too, and they have tried everything else.

i have had pretty good luck with spraying parts down with a heavy coat of spray white lithium grease.

Yeah I've tried grease and oil and lube sprays and other protectant sprays. Finally just decided that cheap engine paint is the easiest since the blocks always get hot tanked again before the shop starts the rebuild process. When I buy a block now I take it apart, get it hot tanked and then spray it with cheap paint.

I use this.....seems to work well

i have used this .... it is very good. i even put it on the
lug nuts on boat trailers! it does not wash off after many times in and out of the water ! it forms a gel like paste when dry... i love it..it works .it claims 2 years inside but it last longer when a block is in an air tight bag

I've always just used grease on the machined surfaces, primer or paint on the cast.
I also bag them, and put them on a small furniture dolly or dedicated block dolly.
I have a used cast 440 crank in the shop, been there for years, merely greased but not wrapped up, zero corrosion.

This stuff works great Fluid Film

https://www.cosmolinedirect.com/

Cosmoline. Its worked for over 100 years and still does.

Gibbs lubricant works very well, so does motorcycle chain lube spray, spray lithium grease, and smearing grease all over it. What ever you use, it is a good idea to check on it every now and then just to make sure no rust is starting.