Small Engines (Lawn Mowers, etc.): gas tank, plastic gas tank, inner tank

Question
I have a plastic gas tank for an outboard that someone left gas in for about 5 years.  The tank is in great shape but there is an awful varnish smell and the inside of the tank has a gooy, sticky residue that can only be scraped out.  Is there something I can wash out the inside of the tank with that will cut through this reside.
Thank you.

Answer
Hey Gene,
If you want to avoid scraping the stuff out all you must do is take fresh gasoline and leave it the for no more and no less than 12 hours. The gasoline should strip the varnish off of the inner tank therefore leaving you with a clean gas tank. Please let me know how this works out.