Redirecting some posts to WP-login if not logged in.

I’ve set up an informational site on WordPress and a custom post type. Now what I need to do is redirect anyone who isn’t logged in to WP-Login when they are trying to view the custom posts or a couple of pages I have set up.

Then when they are logged in redirect them back. I know how to redirect back upon login but not how to redirect there in the first place when not logged in. I know this is very easy to do using sessions outside of WordPress, any suggestions on how to do it in WP?

Many thanks