Hello Kevo,
If you are clearing away surface rust then it's a good idea to use a metal prep solution to completely de-rust your panels. PPG makes a good product and believe it or not, this stuff is actually affordable. It is DX-579 (This product super cleans your bare metal and removes any small amounts of rust that will haunt your finished paint job later in the form of small bubbles/blisters!) and is mixed with water according to directions, sprayed onto a red scotchbrite pad (wearing chemical resistant gloves is an absolute must!) and scrubbed onto small sections of properly stripped bare steel panels. A damp towel is then used to wipe off the cleaning residue and a clean dry towel to finish drying. Getting the metal dry correctly the first time is the key to happiness and if not completed dried, will leave streaks of faint flash rusting on the panel and will need to be recleaned. That is why you do small, managable sections and take your time. This sounds like a lot of trouble but it is well worth the hassle. Just because you don't initially see the rust after stripping your panel does not mean it is not there. I have seen several paint jobs blister due to improper and perhaps rushed metal preperation.
I typically epoxy prime my cleaned bare steel in sections (rear quarter, door, etc) after metal prep to lock out moisture. Even if the panels require body work, I epoxy first because it's the only way to protect the panel from moisture penetration prior to body work, especially if you live in an area that receives lots of precipitation.
For the panels which do not require body work you can spray on some high build primer after the epoxy is sprayed (same day preferably) and begin block sanding with the longest straight block the panel can bear. Durablock sells some decent blocks up to 36" and work great.
If you really want to check you quarters for straightness, grab a 4-6' level or an absolutely straight piece of steel stock and place it against your quarter panel. Look for pockets of light. Especially above the wheel arches. It is difficult to check for straightness by line of sight and even by feel on such a long panel. Who knows, you may not need any filler or you may need a skim coat to get it really straight. Either way, use the longest, true straight edge you can find and routinely check your work as you go, you'll be glad you spent the extra time on metal prep and double checking.
Eastwood sells a great product called internal frame coating which stops the rusting process and seals the steel for good. Just make wure you have at least wire brushed all of the loose bits of rust off, vaccumed, and wiped down with a good solvent prior to applying. Of course, I am talking about exposed internal sections of your frame rails after removing your floor pans. Just make sure that you do not coat the flanges where welding is to occur. Those need to be thoroughly cleaned (preferably sandblasted) with wire brush, sanded, metal prepped and then shot with a thin coat of weld through primer.
I hope this helps a little
