I'm doing some hanging knee raises and honestly, they are not hard at all on my stomach or legs, but the problem is that I cannot keep my hands up there on the pullup bar, they are constantly slipping and this is preventing me from doing multiple reps — I get to 15 reps per set max when I really feel that my stomach and legs could handle a lot more. I've tried using gloves, which actually make my grip worse, and I've also tried using flour to cover my hands in it, but neither of the mentioned helped... Can anyone help me with this?
So you either have weak grip or your hands are actually physically slipping. Both very fixable issues, so don't be discouraged!
If it is ACTUAL slipping, then there's a few things you can do. First, flour doesn't work NEARLY as well as just pure chalk. You know the stuff that olympic lifters use? You can get some at any supplement store or sports store for at most $5. Just thoroughly rub it on both your palms and you should be good to go. Flour really doesn't compare to chalk in my opinion, so FIRST thing you should do is go invest in some.
Next option, change your grip. Try using a hook grip, where your wrap your thumb around the bar then wrap your thumb with your hand (should google a picture or something). This will hurt your thumb at first but over time you'll get used to it and you won't slip this way. You could even try a false grip, hanging off of your palms like a monkey.
Next option (and most effective) use lifting straps. There is absolutely NO WAY that you could slip using these. Just wrap the straps around the bar and tighten the straps and you could basically hang off of the bar forever until your lats give out. You could also get these at any supplement/sports stores or the internet. The one downside to these is that you won't get to work on your grip strength at all, but if your main focus is core, then you can and should do this. You can work on your grip in other exercises.
As an additional note, gloves won't actually do anything for your grip at all. All gloves do is basically help with your hands not hurting and prevent calluses. I definitely don't recommend gloves so your hands can toughen up on their own.
Now, the above 3 are basically guaranteed to prevent further slipping provided you do them right, so try them. Now, the only other explanation is that you have a weak grip...like half the gym population. Now, if you don't care about this, then use lifting straps. But if you do actually care, then you need to work on it. There are lots of grip workouts that you can do that you will online. But the simplest way to get better at this is to just practice it. Your forearms are used everyday so they need a different kind of stimulus. Also, bodyweight exercises are more motor function oriented rather than just basic muscle strength. This means if you want to be able to hang off of the bar without trouble, then you need to just practice hanging off the bar. Start with 3-4 sets of hangs on the bar as long as you can, and strive to improve the duration/sets with each workout or week. You can add on reverse/hammer curls to work on your brachialis, as well as wrist curls to directly work grip. You should also try to incorporate some towel pull ups in order to really improve grip strength. This is where you would hang a towel (or any cloth really) off of a bar and either hang off it or do pullups.
To fix this, you just need to be able to diagnose yourself.
So basically, if it's actual slipping, then use real chalk, lifting straps, or a hook grip.
If it's just pure grip weakness, just practice. Add direct hanging and pulling work and work on the primary muscles involved such as the lats, biceps , and forearms. Research some grip workouts, everyone is different and it's up to you to find one that you enjoy and works for you.
If you don't care at all about grip strength, you can just use lifting straps.
If it's a mix of both weak grip and slippery bar, use either chalk or a hook grip.