Just a quick Q… I have a petrol tank with a few dents in it, and would rather fill them than buy a new one…the only question is what do I fill it with??
FanQ
Petrol?
What’s your tank made of?
Ah so thats what I’m doing wrong!!
Its a metal one!! lol 
If they aren’t to deep use standard polyester filler, it’s good to use bridging fibre glass on bare steel, then sanding filler.Put a couple of thin layers rather than big patch at once.I’ve got some left after filling my tank if you need…
There’s something like this on the market…
http://www.amazon.co.uk/Plaspad-79-Repair-Metal-560ml/dp/B000TAY0NE
I’ve never used it though, so can’t comment on how good it is.
Thanks guys!
There is one big-ish dent…and a few smaller ones. I want to fill them over before I think of paint jobs etc!
I’ve used P38 in the past with good results.
Maybe The Grim-meister might impart some of his wisdom;)
I’ve used thin layer of this on metal:
and this to reshape the tank:
Can’t find filler but have got almost full tin of unnecessary fibre left.
A profesional will always prefer doing the whole job rather than after someone has had a go themselves and done things detrimental. Sometimes it costs the customer more when they’ve only tried to save some dosh ‘by helping’ 
Lol…I like that…a polite way of saying leave it well alone Gary!!
p38 will work, depending on how deep and the size of the dent then fibre glass would do it then fill over the top and sand down, i have had to repair tanks and fairing all the time, my fzr tank was pretty buggered but now after a good few days working on it, and fillering the dents i know have a perfect tank and i sprayed it my self
If the dent is a largish dent then its best to take it to a body shop to get the dent pulled before filling. They use stud welders wereby they weld a couple of pins in the dent and pull the dent with a slide hammer. When you get a dent it distorts the area around it, so when you pull the dent it corrects the distortion. Then all you have to do is apply a little filler.
Hope this helps