I would take the sand out of it , sand gets compacted and goes hard as a rock . I've got mung beans in mine , they're cheap and you can get them from any indian grocery store.
They have a good feel when you hit it , its quite firm but still have enough give in them to absorb the shock . I think almost any type of grain , peas , seeds etc would work just as well . I heard that Wong Shun Leung had ball bearings in his one.
I think the greatest benefit of the wall bag is for your stance , as for the conditioning of the knuckles it seems to wear off on me in about two weeks .
Meaning if I use it a couple of times a week I can punch it 400 times each hand without any abrasions , but if I don't touch it for a while my hands start to soften up and next time I use it the knuckles will start to bleed after a short time.
I think you will mainly want to train your short range power on it , practice hitting it with palm strikes and punches with your arm in the optimum angle ( bent elbow position) with your loose fist a couple of inches from the wall bag .
After a few of those on each hand , halve the distance the fist is held from your body , move in closer and make sure the fist is still only 1 or 2 inch from the wall bag. After some reps on each hand now bring your fist back till it is one fist distance from your chest , move in close to the wall bag , fist still 1 or 2 inches from the wall bag and practice your strike from there .
Follow this protocol with your palm strikes as well . It might sound strange , but initially don't try to put any power in the strikes just relax as much as you can and focus on punching through and beyond the wallbag , try to just firm the wrist on impact not tense the whole arm up .
Any tension will provide a conduit for the force from your strike to travel back into your body which means force is wasted and not penetrating the target properly. Pay particular attention to the thighs and the shoulders being relaxed and staying sunk down in your stance .
After you finished the close range stuff you could just finish off with a couple of hundred chain punches at medium pace with good form , concentrating on relaxing your shoulders and mentally driving those elbows forward .
To add in a bit of speed work to it I like to do a flurry of about 3 to 5 punches on every 10th punch or so to get the heart rate up a bit.
Oh and don't practice elbow strikes on a wall bag the force going back into your body can cause injury to your brain by shaking the brain stem.