If I prime it and paint it with 2 coats, can I use interior grade plywood outside?
Short term, yes.
Long term, no.
Reason is exterior ply has two major differences from interior ply.
Exterior has less voids(open areas where moisture can collect) and is made with a water resistant adhesive.
Of course you can use it outdoors. Depending on the rainfall and humidity and ground contact, it will last a few months to several years.
If replacing whatever in a short time is acceptable to you, go ahead.
Make sure you finish both sides the same way or it will warp like a pretzel.
i have sen 3/8 inch warp from the first coat of latex primer on one side and then nicely flatten out when the second side is primed.
I think I will just use the plywood for an interior project and get something made for outdoors. When I bought the plywood a few days ago I had no clue there were interior and exterior grades.
More than you ever wanted to know about plywood grades and specifications...
Here is a link that might be useful: Engineered Wood Association
Mike, that was very informative thank you.I had no idea. My piece of plywood is not marked interior or exterior but when I went to the company website it only described the red oak under interior plywoods it offered.
Hardwood plywoods (like oak) are not normally available in Exterior grade (especially off the shelf). Even if you purchase exterior grade plywood make certain it is well sealed especially the edges (fill the voids of you can). Just because it is exterior grade which basically means exterior grade glue, it will still delaminate if moisture gets in between the veneers.
Typically hardwood plywoods like oak is going to be for interior use. There are marine grade hardwood plywoods, but you'd have to go looking for them and they're definitely not going to be at your box big home improvement store.