I cut several holes in mine and hand cleaned the varnish out if it with sea foam and a rag. Orange as that is the entire inside probably looks like that. You wont get it super clean with out cutting a hole in it to access the inside. A larger tank will have several baffles so that means cutting several holes and then welding them back up. It is not worth it unless it is a solid tank and honestly not worth it unless you can weld it back up yourself. I will add a link of what I did to mine below:
http://www.classicseacraft.com/commu...ad.php?t=29477