I applied a Herculiner last weekend, since I don't have a compressor, a roll-in liner was the path for me and Herculiner seems to have the best rep amongst roll-ins. What I thought would be a 2-day job ended up being a 3-day job because of seized bolts and a lot more rust than I was expecting, especially under the roll bar forward mounts.
Regardless of if you go roll-on or spray-on, take your time and prep the tub correctlly. With my tub, I wire brushed (using a drill and a wire brush) all the rust off I could, then degreased using a biodegradable degreaser. From there, I used the POR15 "Metal Ready" phosphoric acid rust remover. After I hosed that off and let the tub dry, I applied a generous coating of POR15 to all the rusted areas. Once the POR15 dried, I roughed up the entire tub with the scotchbrite pad that comes in the Herculiner kit then wiped the tub down with acetone. I masked everywhere I didn't want the liner to go. It's a great idea to use those foam earplugs to protect all the bolt holes in the tub. I cut small pieces of masking tape, which worked, but were time consuming to cut and remove, wish I had used the foam plugs. Once you're done with the masking, you're ready to apply the liner. The Herculiner kit comes with a 2-in paintbrush to use in the corners and other places where the roller won't reach. Since you're supposed to 'dab' the liner on (i.e. don't brush it on), I think it would work better if you used one of those short round brushes they use for stenciling or applying stucco.
I found I used the brush quite a bit more than what the instructions suggested...which, I think, helped add uniformity to the coating.
Here's a pic after I had the first coat applied.
Here's what the Jeep looks like after the 2nd coat was dry and I got the seats/roll bar/console back in:
The second coat really added some depth to the liner. The Herculiner also did a great job of filling in the rough spots where the rust had eaten into the sheet metal under the roll bar's forward attach points and the floor pans. Even with a generous second coat, I still have about an 1/8th of a gallon left in the can.
So far, I've been really happy with the Herculiner, we'll see how well it holds up. I did notice the black color makes the interior seem smaller/more cozy than when it was blue body paint and black seats. Strange how colors can affect our perceptions!