Without watching you work or seeing how you put the paint on it's difficult to pin it down to one thing. I can only suggest stuff you can check ?
Are you sure you're washing the shells thoroughly and not leaving any of the mould release in there ? This would effect it.
The the paint getting more of a build up in that area as you spray ?
Are you drying the paint too quick when you put it on or if the above is happening with the paint getting too thick in some area's is it drying all the way through or is it just the top drying ? If it is that when you put the next coat on the inside still won't be dry, it will take ages to dry out because you will have coated it up and in effect sealed it. If this 'wet' bit then dries out it will cause the top coat to crack ?
That's all I can really think of and I'm not very sure it's any of that ? Sorry i can't be of more help.
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