Use redoxide/zinc primer followed by good quality stone chip, the stuff
that dries hard  than paint body colour with brushing enamel, any paint
mixing place should be able to mix some to your colour scheme. You can
waxoyl afterwards if you want to.

Hi

Just welded some patches on the underbody of our open classic, whats the
best way to treat them.  Hammerite then waxoyl or underseal (on top of or
instead of the hammerite).

thanks

Gary