I use galvanized metal containers often. They tend to get hot in the summer, but no worse than plastic. Your pine mulch/compost mixture might be a little heavy, especially if the galvanized tubs are shallow. Look for the "Container soils and water in containers..." post for an insane amount of information on this topic.

15 gallons should be a good size for one tomato plant. Blossom end rot will be a bit of a problem--minimize this by careful attention to watering (not too much or too little).

Painting is not necessary--a good galvanized metal container should last many years without rusting.

Jason

Truthfully I'd be a little leary about planting edibles in galvanized tubs. I too use them for planting flowers and and container water gardens. I'd be worried that chemicals/materials used to do this process could leach out and into your tomatoes. I've container planted tomatoes for years in large planters and always had sucess. As with all my containers I use a commercial potting mix and just feritize according to whats being planted in that particular planter. I was once told too use a vinager/water mix in galvinized tubs and such and that should neutrulize the metal on the inside of the tub. But I would really double check with someone to make really sure. It beats risking the contamination of your tomatoes of the metal does leach. .........mike z5b:ohio

Using vinegar, or some other acid, would be about the only way to leach the zinc from the galvanized coating.

I have been wanting to use galvanized tubs and I was interested in your posts, and I also found useful info from different points of view at these two articles: http://gardeningwormcomposting.com/2011/01/16/is-galvanized-steel-harmful-to-the-soil/ and http://www.ehow.com/how_5730657_garden-galvanized-tubs.html

Here is a link that might be useful: Using Galvanized Tubs?