I frequently get asked (making it an official FAQ) how I do that exquisite crackly finish on my paintings. Alas, a magician never reveals her secrets. Just kidding. Crackling is putting the icing on the cake, it is a real treat - I would love to share.
My dad discovered this product and quickly addicted my sister and me when he gave us each a pair of bottles for Christmas. The product, by LeFranc & Bourgeous (very French), is a two step process. I usually finish the painting with a coat of Liquin which makes me sneeze (another topic for another day). The first step is to apply an even coat of the ageing varnish. You then wait two to three hours, depending on how fine you want your crackles. The drier the ageing varnish, the smaller the crackles, yum. You want tacky, but not sticky. I usually wait till the tack is almost gone, I like 'em fine. Here comes the mad skills. You only get one swipe to apply the cracking varnish. Thin, but not too thin, streak free, but don't fuss with it, fast, but keep it even. This layer is water soluble, so if you mess up you can wash it off with soap and water. But don't, it's a pain in the @%$. Wait a few hours, or a day, and then you can make the magic happen. Paint the whole thing over with an oil paint-asphaltum, sepia, and transparent oxide brown are my favorites. Then you rub with kleenex, q-tips, those cotton things the dentist packs your teeth with (thank dad for that one too!). It's so much fun, you can deepen shadows, pop highlights, bring down the tone of an area, leave the edges darker to make it look old, and on and on... It's so much fun. When that is dry, I spray with a gloss damar varnish... and voila!