Treating rust spots on a petrol tank

[andy120t]

Some years ago I stripped a petrol tank and left the metal bare - result, lots and lots of tiny spots of rust. I rubbed these down and sprayed a single coat of undercoat on it and the spots reappeared through that. Doesn't seem to have become much worse in 5 years, just lots of bumps and little rust spots through the paint

I'm planning now to strip and repaint this for use on a project - does anyone have experience of how I should treat the metal / anything I can paint it with to stop these coming through again and to let me kave a nice smooth finish?

thanks
Andy

[Menno]

It depends on what kind of undercoat you used.
Undercoat is used to get a better grip for the paintcoat and often also works as a filler.
However most undercoats don't work as an isolator. That means moist can still penetrate through the undercoat onto the steel.
Hence rost.

So sand your tank again.
Put an undercoat on it again and colour.
Than finish with a clearcoat.
The clearcoat preserves the tank against water (and rust underneath) and if you use the right clearcoat also against gasoline.

Be aware that clearcoat often needs to harden over a longer period (3 - 6 months) to get is final strength.
So it will be gasoline resistand but in the beginning not as resistand as it will be after half a year.
Whipe excess gasoline on new paint immediatly off and be careful.

In the old days clearcoat was much stronger because of the use of stuff like lead in it which is forbidden nowadays.
The lead in the paint however is the main reason why all of the colours on Hondas of the 70's faded so quickly in the sun:
The lead melted.

[andy120t]

Cheers - that fits with my thoughts which were that I should do the job properly; undercoat, paint & varnish it is.
Andy