I put down Hurculiner in my sub from the firewall just past the end of the bed.
My experiences with abrasives:
1) Sandpaper is a balancing act, it is incredibly easy to cut down to bare metal quickly, unless using a very fine grit.
2) Using abrasive pads like scotchbrite or whatever totally blows. WAY too much work for the poor level of abrading, and way too expensive for how much work is involved and how far one pad takes you.
3) HARBOR FREIGHT STEEL WOOL. I kid you not. I did the entire bed with a pile of scotchbrite pads, then did the entire cab with 2 little baggies of Harbor Freight steel wool balls. The steel wool dulls up the paint FAST, evenly, and very rarely, if ever, cuts down to bare metal. I even had a few left and went and re-did sections of the bed that I couldn't get done with the scotchbrite pads. I think the baggies are like $2-3 apiece. 3 bags would probably get your k5 done, and run you less than 10$. Buy a bag, test it out, and tell me I'm wrong.
I put down 3 gallons of Hurculiner in The Sub. It got me just under 4 full coats. To be honest, the Hurculiner looked like crap until I put down the 3rd coat. To prep I ripped out all of the carpet and insulation, vacuumed and cleaned, dulled all of the paint and self-etching primered bare spots, washed with TSP, wiped everything down with Xylene/Xylol (I was amazed how much stuff I was still able to wipe off), then masked everything. Don't use solvent after you mask because you may ruin the adhesive on your tape.
Biggest lesson that I learned was that good prep takes way longer than you could ever imagine, and if you plan to roll on, you need to make your masking tape more than 2 strips wide. I went with 2, and still got that stuff everywhere.
Oh, and Hurculiner soaks through your clothes, and Xylene makes your skin feel like it's on fire. Temporarily.