Should I use modals for advertising?

I have a website where I want to advertise a specific product/service. Is it OK to use a modal that grays out the rest of the screen to show this product, or will this turn away visitors?

Edit: The modal I am planning on using will appear in the center of the screen and overlay the rest with grey. 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 by clicking outside the box. It it mostly unobtrusive.