My nephew offered to stain my deck. I did not pick up the stain and trusted him to pick it out. However, he stained it with Minwax which I understand is for interior staining. Since I love the color is there a way to cover this with a waterproofing material, an if so what should I use? He is gone for the summer so I would like to do this before he returns..

I would check with a pro paint store in your area. Different regions and climates have a variety of issues from temperature extremes, intense dryness to humidity. And solar damage is always present if it is open/exposed to sunlight.
A paint store like Sherwin Williams or Benjamin Moore will be better able to recommend an action to help solve the issue.

We do have a Sherwin Williams store in town..will check it out thanks!

Does Minwax have any info available online? Many of the manufacturer do - in order to promote their products.

Personally, I would start there to possibly save a trip.

Good Luck!

Don't know...why didn't I think of that! Thanks!