[John de la Vega]

Not smart! It might not take that long to crack, we're talking years counted by fingers on one hand! Of course there are many factors affecting the possible cracking, the main one being how long the lean was painted after the fat was applied. There are several Sargent paintings that exhibit cracks, and did so early, I'm sorry to say. Most of these have been restored, but a few fell through the cracks (silly me). He, unwisely, often reworked thick passages (having a tendency to use a lot of paint practically from the start of the painting) with thinner layers.

These days oils dry faster than even twenty or thirty years ago, so if you apply the lean 2 or 3 weeks after the fat (unless it's VERY fat), chances are there will be no cracking, Liquin or no Liquin (no medium is guarantee of cracking prevention).

Lean over fat is also not recommended in hot, humid climates, if there will be temperature changes where the painting resides.

If you must glaze or apply thin layers on top, paint the lower layers with acrylics (you can really build up lights this way for transparent upper glazing with oils using Liquin or other mediums, the more bodied the better). You may also use alkyd oils, but if you do - and they're fine for underpainting - wait at least 3 days to apply thin over fat.