Why is Model Master paint cracking?

My guess would be that the paint beneath the MM was not fully cured. What type paint is the Tamiya? Acrylic? Enamel? Lacquer? How much time did you give the Tamiya to dry before you brushed on the Model Master?

As for a fix, you could strip the model entirely and start over. Do a Google search for various methods on that. If it were my build I might try to give it a few days to completely cure, then lightly sand it with fine sandpaper, wipe clean and try the second coat again. Depends on the kit, I guess.

Good luck!

Chris

I'm also thinking the tamiya paint wasnt fully cured and reacted uder the MM enamel. Take some thinner and use a small plastic ontainer and a soft brush to remove the enamel. Let it cure for at least 2-3 days then try agan.

Paint brushed on too thick can cause cracking. So would humidity depending on where you are. 

IMHO the mixing of paint types (in this case enamel over acrylic) is always risky.  Yeah, I know some folks get away with it regularly, but there is still the risk.  I had an episode like that recently.  I seldom do it, and seem to have more than my share of problems.  I think I am about to resolve never to do it again.

"Enamel over acrylic" is a no-no.  Enamels attack acrylics. 

I sprayed enamel white and black invasion stripes over acrylic camo on my airfix 1/24 scale Hurricane about ten years ago,and I am still waiting for it to crack.I used Tamiya primer w/Tamiya camp ,and enamel testors paints.

Tamiya camo 

I think a lot depends on how thick you lay it on, though. I've gotten away with laying one supposedly incompatible paint type over another just because I tend to spray my coats pretty thin. Misted, almost. My thinking has always been that if one lays on a thick coat, the solvent within that coat takes longer to dry, so it has more time to react with the coat below it. That and the simple volume of solvent will be higher as well in a thicker coat.  Anyway, just my .02. YMMV.