I pulled out my fuel tank (plastic) yesterday to empty out the old gas. And at the bottom of my tank was a dark reddish/Brown goo. What is this from?

I plan on taking it to the carwash to spray it out, then leave it out for about 3 or 4 days. I'm guessing it should be dry by then, or do I need to do something like put some alcohol in it to absorb the water?

Sounds like varnish from old gas. Get some Chem tool and clean it out then dispose of it properly. Water won't really help.

It is from gasoline turning it to what is commonly called varnish.

I do not know if a carwash will remove it?

I used a combinations of BB's and Ball Bearings (Sling Shot ammo) I poured about 2000 of each added 2 Gallons of Paint Thiner and drove it around in the back of my truck for about a week. That got most of the sludge buildup. I then emptied it out added gas and gas system cleaner. (water remover and cleaner)

this stuff is coming off pretty easy... if I wipe my finger across it it comes right off. The tank bottom isn't covered with it or anything its just a few spots. I'm pretty sure a pressure washer from the carwas will remove it easily.

Do i need to use gas and a gas system cleaner to remove the water? Like STP or sea foam?

After it's cleaned out I would use some Seafoam in the next tank or two.

Since your saying that it comes off by wiping, then more then likely a car wash will do the trick. Make sure you remove the fuel sending unit(if it has one) as it does not like water. I would use a can of carb cleaner or alcohol to dissapate the water.

Ok ... Thanks for the help.