Should I use modals for advertising?

I have a website on which I want to advertise a specific product / service. Is it ok to use a modal that displays the product in gray on the rest of the screen or will it divert visitors?

Edit: The modal I plan to use will appear in the center of the screen and will cover the rest of the gray. The box will contain a coupon code for $ 50 off and a small Google ad. The box can be easily closed with the X button in the corner and clicking outside the box. It is mostly unobtrusive.