My outdoor Built in BBQ has a lot of rust, but NOT rusted through. I want to get rid of the rust and then treat it and paint it to prevent further rust

If the rust is on the exterior of the grill, remove the rust using a steel bristle brush and then smooth it down with sand paper.Spray a light coating of High Temperature Grill Paint (you can find it at most hardware stores and home centers) This first light coat, will act as the primer coat. After it thoroughly dries, apply 2 more light finish coats allowing each coat to dry, before applying the next coat.
Based on the air temperature and the humidity level in your area, wait at least 3 days before using the grill.

If the rust is on the inside of the grill, just removing the rust will generally take care of it As most grill tubs are made of cast aluminum and don't require painting. What may appear to be rust is flaked on food and grease. In which case, a plastic putty knife and a roll of paper towel will take care of it.

And finally, investing in a grill head cover (made for built-in grills), will really help in not allowing the grill to rust again.

Hope this helped you and Happy Grilling!!