i'm relatively new to modelling, but of the three that i have used tamiya clear over tamiya spray paint, i waited 24 hours after laying down the final coat of spray paint to start laying on the clear. i've only been making models for about five months now, but within that timeframe no cracks have appeared, maybe some will later on or maybe not, who knows? but for the time being it seems to be just fine. hope this helps a little. B.