Is there a product available that will allow me to refinish/re-enamel
a worn porcelain sink? (it no longer has a glossy surface and
therefore always looks dirty)  Ideally it would be something that i
could spray or brush on, and could find at HomeDepot/Lowes/etc., but i
am also willing to hear about products that are more complicated to
use or harder to find.

Dear D Wee Weee, 

There are several products that can resurface a porcelain sink. It
happens to be the coincidence, that I was at my local DIY/Hardware
lately, and got answers on the issue.

"The TUBBY kit can be used on cast iron (enamel) baths, steel,
plastic, fibreglass and acrylic baths. It will even cover ceramic and
porcelain surfaces which means the entire bathroom suite can be
transformed for relatively little expense.

Tubby can be used on bathroom sinks, toilet exteriors and bidets. " 
http://www.tubbyusa.com/ 
(For UK/Ireland orders: http://www.brandonmarketing.co.uk/ 

If you're a New Zealander, Tonercote might be a solution for you. 
http://www.tonercote.co.nz/index.html

Another product is Armorpoxy <http://shop.store.yahoo.com/armorpoxy100/bare.html>

If you live in Opelika, Alabama, area, this store offers seminars on
bath/sink refinishing (including handling a porcelain sink):
Tub King 
<http://www.tubking.citymax.com/page/page/207910.htm> 


Further Reading
===============
"How to Refinish Tile, Sinks, Countertops and Appliances
A Step by Step Guide " - Plumber Pages
<http://www.plumberpages.com/baths/refinish.htm>

The site DIY Doctor provides
<http://www.diydoctor.org.uk/projects/bathrepairs.htm> advices on
various facets and needs of bath/basins repair/resurfacing.

The Australian site Home Ideas
<http://www.homeideas.com.au/showprodinfo?id=343&state=6&cat=4>
provides information on Thermo-Glaze bathroom reglazing material.

Boston Globe Q&A on Bath Reglazing
<http://www.boston.com/globe/search/realestate/hotton/reglazing.htm>.

I hope this answered your question. I used the terms "bath
refinishing"/resurfacing/reglazing and added the term "porcelain".

If you need any further clarification on this answer, please let me
know before you tip/rate it.

dweeweee...

I hesitate to post this in the answer space until you've had a 
chance to try this on your particular sink, but there is a pool
cleaning product called The Stain Eraser which might just do the
job of resurfacing your sink. Until recently, I had a calcium
deposit ring around my toilet in which black algae had taken root.
I bought this product in the pool supply section of my local
grocery store, and it took it right off. I had tried everything 
prior to this, so I'm very happy. It rubs out rust, calcium
deposits and other stains.

Now the link below shows the product on the end of a big plastic
stick, like a lollipop. The store I went to offered this as well,
but for something like $13! The same product comes in a block
about an inch square and 6 inches long (so much more of the product)
for under $4 - at least they had that at my store. It's a cheap
investment for the potential results it can give you. It's kind of
like a big pumice stone, but it's made of slightly softer material,
and will wear away as you use it. Here's the link:
http://www.thestaineraser.com/

If you give it a try and it does the job, let me know, and I'll
post this as an answer.

sublime1-ga

Dweeweee~

As someone who moved into a house where the previous owners
"refinished" the sink and tub, I cuation you not do it in your own
home. The "refinish" cracks and flakes off, particularly when soap is
laid on it...but sometimes even when it is soap-free. Oy!

Kriswrite

sublime-

Sounds like 'the stain eraser' takes rust/calcium/etc. stains off of
surfaces, but doesn't actually resurface the sink, which is what i am
looking for.  Is this assumption of mine correct?  My sink has no
'gloss' left to repel stain, the smooth enamel surface has mostly worn
off and a gritty, more porous surface is left (I read that this is
probably from years of abrasive/acidic cleaners).  Any other ideas?

dweeweee...

Well, it's hard to be sure until you try this product. After using
it on my toilet bowl, I used it on my bathtub, which looked as
you describe your sink, with what appeared to be a worn, rough
surface in spots. What looked at first to be a badly worn surface
turned out to be a fine layer of lightly-stained calcium deposits.
When I used the Eraser on it, I was delighted to see a shiny surface
emerge as I rubbed and rinsed off the grit.

But if you're positive it's really worn out, I suppose you'll
need to explore other options. Though I personally don't know of
a refinishing method for porcelain which is worth the trouble
compared to replacing the sink, I'm sure someone will have some
other ideas.

sublime1-ga

dweeweee...

I was at the grocery store today, and found the name of the
actual product I used. While essentially similar to the 
'Stain Eraser', it's called 'Pool Blok':
http://www.recsupply.com/myweb/products/poolblok.htm

sublime1-ga