Over the last few weeks, I've looked at a few older oil paintings of mine and thought they needed a bit of an update. The technically safest way to do that is simply to repaint them from scratch; but if the paint isn't too thick, you CAN add fresh paint on top of a dried oil painting, provided you don't use any thinners with the new paint (a bit of oil painting medium, such as that made by Winsor and Newton, or even Liquin, is unlikely to hurt). People are very afraid of cracking - with good reason, because it can wrench the paint film apart (more especially on a flexible support, like stretched canvas) - but the careful addition of subsequent layers of paint, eg just adding brighter colour to the impasto'ed highlights that may not have been quite high-lit enough, can transform a dull painting. Painting an entirely new picture over an old one is riskier - although most of the old masters did it, with varying degrees of success - and unless in desperate poverty with nothing else to paint on, I wouldn't do that. But don't think you need to just put up with - or throw away - an oil painting which now strikes you as dull and boring. Use the paint with as little medium as you can, and certainly not drying oils or turps: and yes, it might crack. But then - if your painting now seems desperately dull and uninteresting, and you can't sell it or even get much praise for it, what have you got to lose? Such is my reasoning, anyway: better to use bright, brilliant colour from the outset rather than mud, but if you haven't, and your painting is just boring, zhoosh it up a bit: you might well get away with it, other things being equal, and a little bit of surface crazing isn't so disastrous, even if it happens - which it probably won't.