I bought a creased gas tank for $75 off of EBAY.  It came with no fuel pump (as expected) and everything looked perfect (except for the hatchet blow in the side.)  I removed the emblems and began to sand off the paint and fill the crease with my absolutely favorite stuff in the whole wide world: Fiber reinforced filler.

This stuff is so awesome.  Dynatron makes the identical stuff.  It is easy to work with and sands easy.  Plus, it is strong.  I laid in some fiberglass mat with some polyesther resin first, then I filled with the bondo-glass.

I figured while I was at it, why not fill in the seam too?  So, I did.  Here is a pictorial of the progress:  

Slow going, but steady progress.

Now we are getting close.  The curvature is not yet quite perfect, but approaching the correct radius.  For this glaze coat, I use Dynatron, Putty-cote #592

This stuff is recommended in the Paint 101 link provided.  Puttycote works well as a second-to-last glaze.

I found the Evercoat glaze coat is awesome and goes on like melted butta'.  I use the Evercoat as the last glaze coat.

After sanding, you can see the high spot created by the crease.  For the most part, I pounded this down, but some "peak" still remained.  So, its time to add more fiber reinforced filler:

This helps to level things out as the Dynatron glaze is not designed for high build.  

Now the ridge is gone and the radius matches the other side.  It is time to prime and paint.

Using HOK Catalyzed primer.

The gas tank seam is gone too.  Click up one level to see the remainder of the paint job.  click here