If you don't cover the cans, they'll be fine. Using them as collars around plants is an old trick. Just be sure to remove them before the plants grow too large. 3 days won't be nearly long enough, probably a month or so should do the trick.

I wouldn't bother painting them either. The metal finish will reflect some heat, and if you're planting out before the ground really warms up, it should help them.

If you do need to cover them because of a freak cold spell, use rubber bands and plastic bags. That's not good for long term solutions, but for a couple days, it should be okay. The cold is what makes rubber bands brittle.

As Token stated using cans as collars goes way back. I remember as a child helping my grandma put cans around a lot of plants.

Sandy

Great! I didn't save them for nothing. And I'll feel better going to work each day knowing my little babies have at least some protection against the cruel, cruel world out there.
s

Not sure what critters are trying to eat your seedlings yet I find strategically placed bamboo skewers work great at keeping the squirrels from doing their nuisance digging.

-StLGirl