Keep in mind that I am not proclaiming to be THE expert, but if you want a few proven steps that might be of some use read on.....
The most important tool is your shaped sanding block. I use a cork block, which I sand a curve into using an old piece of tubing with a piece of course 40 grit sand paper wrapped around it to get the right shape.
You will need to get your frame stripped either chemically or bead/sand blasted.
Remember that if you sand blast the frame you can't chrome it, as the resulting finish is to rough, and even painting afterwards , you will need to rub down with 240-320 sandpaper.
If your frame has any small dents or dings (most do)that you don't think will compromise the safety of the finished bike you can use automotive two pack filler to repair.
As you can see above, this Bates Bar needed quite a bit of filler.
If there is a ripple or any kind of irregularity on the under side of the top tube or down tube behind the front lugs, STOP.
This will tell you your bike has been in a frontal accident, if the dent is very minor you might be able to still use the bike, but before carrying on consult your trusted local bike shop mechanic and get their opinion.
The Austral Star I am rebuilding at the moment has such damage. As the bike is mainly for display use I have just straightened it myself.
Most frames that I have seen that are 30+ years old have some degree of metal decay,which generally shows up as shallow pitting, mostly very minor.
The tube above is from an at lest 70 yo tandem, and will need quite a bit of work.
In either case, I will use a good quality high build primer, two or three coats depending on the depth of pitting.
It is very important to use a good quality high build primer, as a lot of the cheap products tend to sink back over a period of time, which in short means that after about two or three months your beautiful machine will look like..well you know.
So find yourself a good auto paint supplier, and ask questions.
Frames all painted in high build primer, ready to be rubbed back.
A Bates top tube after sanding back with 240 grit wet and dry paper.
Note the way the high build primer has filled most of the smaller pits.
Paint with primer again, this time dust coat a darker colour over your primer, as in photo above.
This will help show up any high/low spots where you have used filler, or that you might of missed altogether.
This is known as a guide coat, you can use this term for a little credibility at the paint shop, I know I do.
Sand back with 240-320 wet'n'dry paper.
Notice the couple of low spots in photo below, clearly defined by the black guide coat.
After you have fixed your high/low spots, go over frame and use spot putty to fill any remaining little pitting marks, some bikes like the tandem below could need quite a bit, however time spent on these stages will result in a great finish.
Don't fill anything to deep with spot putty, as it tends to sink back, it is only for very shallow pitting.
Block back one more time using your shaped sanding block, this time use at lest 320 grit paper.
As you can see below, the spot putty is very effective.
Prime once more.
Using wet 'n' dry 600-800grit paper this time to gently rub back, be careful not to
rub back to the steel.
Your frame should be ready for paint, which I will go through real soon..it's the fun bit.