The texture of bed liner paint is great for covering defects. There's also this to consider. Bed liner paint is made to build up a real coating. There is some cure time to the do it yourself liner. As curing happens the paint settles and the lines from brushing or rolling just fade together. Roll on is about the same as a spray coat except with little mess to deal with.
Washing the truck seems a simple matter. The wash is a must for a first step. Paint simply cannot and will not stick over old grease or wax which are often found around truck beds. Now how the wax got there may be a mystery!!! The first step to success is to get an automotive wax remover and get that surface really clean.
It's rough abraded surfaces that hold paint the best. So that's what is required... rough preparation. It's rough sanding you need for a surface that really sticks and holds the paint. Scratches hold the paint more so than any really smooth preparation. Get that and you get a better stick and less chance of peeling.
It's possible to get a bed liner kit and avoid the hassle of putting together all the components you need. That saves trying to find the few pieces needed in different places. Herculiner is not the only source for the kit solution.
Get more paint than recommended... A gallon is the most oft discussed quantity of paint. That will cover a full size pickup, but just barely. A thicker coat works better in several ways and that means get more if you can.
Be reasonable. As suggested above, spray on liners put on by the pros are much thicker than what you put on. That's key to the durability of professionally applied liners. Thick coatings are tough to scratch through and tough to harm. Do it yourself bed liners are often plenty good enough but they generally are no match for the toughness of a much more expensive liner professionally put in.
Older truck in need of an improved appearance are often where do it yourself liner get installed and rightly so. Bed liner paint is simple to use partly because it is so thick and easy to deal with and install. It takes some time for installation but the steps are simple and not so difficult.