I wouldnt consider myself a professional but have painted a lot of bikes and a few cars (and a butt load of houses).

Similar to what Dave said.  Looks like you gotta paint those welds anyways so,

1. Lightly clean up the loose paint.
2. Mask an area that includes all the bare metal and some of the yellow stock paint.
3. Sand the bare metal and stock paint lightly (maybe 800 grit), within that masked area.
4. Primer the bare metal (keep the repair part light, primer can build up fast).
5. Assuming you dont want to buy color matched paint. Buy the closest yellow you can get in Dupli-color aerosol paint (from O-reily or where ever). Paint the area with 3 or 4 coats.
6. You will either be left with a ridge from the new paint to old, or you can try to fade it in to the old.  Then use a little polish to make the transition less noticeable.

Theres other options, but thats pretty quick and easy for a spot that you cant see under normal conditions.

I deleted my post - as Batman with his limited experience believes he knows better.  I specifically avoided the use of etching primer, as it has a strong acid in it and I don't like using the etching primer on areas where it will contact existing paint (it works great on bare metal).  But obviously Batman has a superior knowledge in him limited experience and is willing to correct me when I am so obviously wrong.

I used epoxy primer on every piece of my motorcycle......even the fuel tank.  I also used it on the Ninja fuel tank, the Savage fuel tank where I welded up the speedometer hole, and the last 2 cars I restored.  Sanding the metal parts and getting some "tooth" for any primer to adhere is always a good idea and helps with adhesion on existing paint - but if you notice the factory primer/paint is applied to clean metal and there are no scratches from the factory sanding anything before they paint.

I am not why Batman feels the fuel tank is going to get up over 240 degrees when it has fuel in it - but once again I am obviously wrong in suggesting that epoxy primer should be used.

My tank has been on the bike for more than 10,000 miles with the epoxy primer...I guess I should stop riding it as it is likely going to fall off from the affects of engine heat any day now.

I will not post in this thread again - as Batman obviously has it all figure out.  

Once you repair the area I would shoot the entire tank with a 2 part clear coat to protect against fuel damage. You can get 2 part in spray cans with the epoxy in the bottom which you activate when ready to spray.

Nifty product that turns iron oxide into iron phosphate available at Kelly Moore.
A quart is under ten bucks. After you've cleaned it to shiny metal, use it. It'll react with the rust . It may look clean, but some is lingering. And it'll blossom eventually.
Liquid, not gel.
Used to be called
Ospho
Renamed a while back.
It's dilute phosphoric acid.

Get some good masking tape, protect the good paint. Waste tape, put on layers if that's what you need to do to avoid a complete paint job.

Fwiw, I haven't been without some of that in my shop in twenty years. It's just that handy to have around.