Did you read the part in Van's plans book about the oils that are in the alclad aluminum sheets from the rolling mill? The self-etching primers get through that and give a good bond to the metal. The ribs are bare aluminum and will need the protection most of all. Van's mentions a Sherwin-Williams wash primer in the front of the plans book and it worked well for me. I mix it up in a 1-2-3 ratio. The 2 and 3 parts are the Sherwin-Williams chemicals.
The 2 parts is the industrial wash primer P60 G 2, the GREEN stuff with chromium zinc oxide, epoxy polymer, and other stuff in it. The 3 parts of the mix is the R7 K 44 wash primer catylyst reducer, phosphoric acid, MEK, etc. And the 1 part is just lacquer thinner to help it dry faster and not clog up my HVLP sprayer.
For a full load going into the sprayer, I mix up 100 ml of lacquer thinner, 200 ml of the GREEN stuff from the gallon paint can, and 300 ml of the acid from the gallon plastic jug. If you get this stuff from Sherwin Williams, you will need to buy 2 plastic jugs of R7 K 44, and only ONE gallon paint can of the P60 G 2.
ALL this kind of stuff is hazardous to your health, so be sure to wear a good carbon filter respirator and have a clean shave to insure that the mask seals good against your face. Wear good nitrile gloves or other chemical resistant disposable gloves and work in a well-ventilated environment away from any gas furnace or water heater. If you look at my web site, you will see that I always worked out on the driveway using plenty of Van's packing papers to keep the stuff off the driveway.
Check out this priming session on October 11, 2003 to see what I am talking about. http://www.n2prise.org/rv9a025.htm
Good luck with your project and make it last a lifetime, or until you sell it and build something newer!
Jerry K. Thorne
East Ridge, TN.
RV-9A N2PZ (Enterprise)