Train abs with weight or no weight?

A big question I get asked a lot is "should I train abs with or without added weight as resistance?".  This is a very good question as everyone is looking to get those razor sharp abs that are deep cut.  Before we explore my answer I think it is important to address a few other items.
1) Everyone has abs.  Yes, even you!  Even if you haven't seen your abs in the last 20 years, everyone has them.
2) As a follow up to the first item, you need to have a low enough level of body fat to see your abs.
3) As a follow up to the second item, you need to have your nutrition dialed in to get your body fat low enough to see your abs.
Good, now that we got that out of the way, let's take a look at whether or not it is beneficial to train abs with weight.  The answer is yes however, you need to take a few things into consideration.  The body will often recruit other muscles to help accomplish an "ab" movement if you don't do the exercise properly.  If you haven't trained abs with weight and you go grab a hundred pound dumbbell to do your weighted sit ups, you may not see the result you are looking for.  You need to work your abs from the inside out.  Everyone should first target the transverse abdominis (TA).  This is the inner girdle that you don't see from the outside but plays an important roll in core support and tight waist appearance.  This can be best trained by some of your subtle movements such as controlled crunches, kegels and planks.  You should start by mastering these movements and then move on to more mobile movements such as reverse crunches, hanging leg raises, incline sit ups etc.  And finally, after you have mastered the mentioned exercises, you would move onto some weighted movements.  Some of my favorite weighted abdominal movements are Turkish get ups, weighted incline sit up, and ab wheel.  I include ab wheel as I feel it should be included in an advanced category.  And finally, when you are in the advanced category, I would recommend you hit abs 2-3 times per week alternating between your high rep body weight exercises and your lower rep exercises with additional resistance.
So there you have it!  Train your abs with weight but only when you are ready.