Here's the deal w/just rolling new product over the old stuff: If it chips easily now, then your new product isn't going to last because of the crappy product you did not remove...Since the base coat isn't sticking to the bare metal well, all you're doing is putting another layer on that's going to chip off just as easily. I recommend hitting sanding it down again prior to applying the new product.
I used Herculiner with great success and even rolled the bottom of my tub since it was already off the frame. My only complaint about Herculiner is that it is wearing rapidly where my right heel rests (same place all the time cause of accelerator pedal).
MODS: 2.5" Rubicon Express lift, 1" lift greasable shackles, POR-15'd new frame, Chrysler 825 rear end w/355 gears and disk brake swap, solid front axle (no more vacuum!), herculined tub (interior & bottom), Dunlop Radial Mud Rover 32X11.5 R15 on black steel rims, custom hi lift jack hood mount, Ramsey 8K winch, and AEM Brute Force air intake system.
Custom rear bumper in progress, Rock sliders on deck, and then front bumper...all fabricated by...ME!