If the graphics have been sitting for a few years I'd say the glue has gone off. Maybe a bit of clear silicone?
As for the tank, I would suggest emptying out completely and washing out with warm soapy water and leave for a day or two out in the sun with cap off.
Once you have fitted graphics, leave them to sit for about 14 days before refuelling tank.
I have a fool proof way of fitting graphics without using soapy water ( or anything else for that matter) which I'll try and explian.
1. make sure surface is clean...you know the drill.
2. get a nice sharp stanley knife.
3. flip graphic over so you have the back (sticky side ) up.
4. with the knife make 2 cuts the entire length of the graphic into the backing paper about 20mm apart (horizontally is best), but make sure you only cut through the paper and not the graphic.
5.You should then be able to pull the strip of backing paper off, exposing about a 20mm line of the glue surface. The rest of the backing paper should still be in place.
6. now put the graphic into place and line it up, this is easy because most of the backing paper is still in place, once you have it where you like it, then press down on the area that you removed the backing paper, this holds the graphic perfectly in place, while you remove the rest of the backing paper and stick the whole graphic down.
Make sense
