SPF means Spruce, Pine, or Fur. This is what houses are framed with. If you care for it, it will last a life time. We prefer oil-based clear finish. The wood will turn a silvery gray over time. If you prefer a color, you can use a stain. Just make sure you stick with a breathable finish, as wood needs to breathe.
Do not seal the wood with a hard finish, like polyurethane. It can trap moisture in the wood and will rot from the inside out. This is problematic with painted tables also.
Try to keep the table legs out of the dirt or grass. This will cause moisture wicking up the legs (like a wick in an oil lamp). Place the table on a bed of stone, on a deck or simply put small cement pavers under the legs.
Why don't we use treated lumber?
It is our opinion that Pressure Treated lumber has too many chemicals in it to eat off it or have bare skin touching it.
Clean with soap and water. If there is any sign of mold, use some bleach or deck cleaner. Be careful, you don't want either chemical to touch your skin. Then hose it off.
We get our Eastern Cedar from the Thousand Islands. It is a wood with natural oil. Cedar needs nothing to make it last.
We have our own Adirondack Chair and a Kid's Chair that have been with us for 15 years. They are always outside. They are a beautiful silvery grey. Clean them? We don't. If you do, simply use some soap and water.
If you wish to stain cedar, we suggest, once again, staying with a breathable finish. Wood needs to breathe.
A customer mistakenly sealed a Cedar Chair with Polyurethane, and it turned green. It had rotted from the inside out.