I refinish furniture. This time I refinished a beautiful picnic table. I put 3 coats outdoor poly on it. Will that be enough to protect it?
Until the finish starts to deteriorate, yes.
There's no such thing as a maintenance-free wood finish for outdoor use. Paint is the most durable. Clear finishes are much more susceptible to damage and sun and standing water are the main culprits. The first thing to do is make sure the table is not directly in contact with the ground or concrete or in a place where it will sit in puddles of water. The second thing to do is maintain that finish so it never gets a chance to crack with age.
Outdoor finishes usually have UV-inhibitors that keep the sun from damaging the finish and the underlying wood. But that can only absorb so much sunlight before it's useless. So these finishes should be cleaned, sanded and re-coated periodically - often annually. This restores the UV-inhibitors. The frequency depends upon the amount of exposure to the weather and the quality of the chosen finish. If you see the finish peeling or cracking, you've let it go too long without maintenance and might need to do a complete refinish to restore it back to good condition.
I thank you kindly sir. Will take your advice. The product used was outdoor hellsman.