My sentiments are similar to WDORRY's. There are several professional paints that will work with aluminum (POR-15 is not a professional paint system that has been tested to the same standards that the larger companies use).
I have used a PPG product and have had good results (do a phrase search for "frame paint" and "bed liner", there's a lot of good info), but there are other paint suppliers that have good products as well. Check out Sherwin Williams, Dupont, and Sikkens "Automotive Refinish" suppliers.
My advice is to look to a 2 part urethane epoxy primer that will adhere to aluminum. The data sheet will call out several properties (what surface the paint was designed for, it's corrosion resistance, UV resistance, harndess, temperature limitations, etc.) as well as what coatings are suggested for top coats. Make sure you get a data sheet!!!
Since you are using Durabak, none of the paint suppliers will guarantee chemical or mechanical adhesion (they would have to had tested that particular combination, which isn't in their best interest to do so). However, they may be able to give you suggestion on which primer to use. They may give you a concern with the nature of Durabak being a 1 part coating. I would suggest a 2 part urethane coating like SEM or gator guard to name a few. They are better in every aspect.
You can get an undercoating gun from Harbor Freight pretty cheap (like $10), it would make the bed liner application much easier, faster, and consistent.
Some say that prep is the most important step, but research shouldn't be overlooked. It looks like you found out the hard way with POR-15 (my sympathies, I'm sure you spent a lot of time on the underside).
Good luck!