I stayed with the nail polish (opi base coat) to start and seal mine mainly out of a lot of trial and error using it. My first few times cracked like mad but I learned if I put a super light coat on first (to give the paint something to grab onto) and then a super light coat after its very flexible and hasn't cracked.
With that said... someone mentioned the spray on sunscreen, I have not tried that since I don't use it but I guess you could try painting your nails, letting them dry and then spray the sunscreen on and see if it gets sticky?
I just finished 2 of my 4 bands, this is teacups (with no border) and peter pan. I have the worst time taking photos of these because of the glare!!!
Oh I did find something that REALLY helped me with some of the smaller details (like eyes), I used a Staedtler pigment liner (copic/sharpie could work too, this is just what I had) instead of my normal Golden's liquid acrylics and it worked great. Now, I tried this directly on the band and it just rubbed off, but if I put it on the acrylics it was fine and didn't smudge when I put the sealer on.