Why the platinum cure silicone don't cure?

Platinum cure silicone can be inhibited (prevented from curing) if it comes in contact with certain materials. (Note: To seal a model, use acrylic paint–not latex or polyurethane.)

For proper cure, avoid the following materials:

TIN CONTAINING COMPOUNDS
Condensation cure silicones

SULFUR CONTAINING COMPOUNDS
– Thiols 
– Sulfides 
– Sulfates 
– Sulfites 
– Thioureas 

Latex (gloves, tubing, paint, etc. Note: some aerosol paints contains latex) 
Wood
Natural 
Rubber 
PVC (plasticized) 
Neoprene rubber 

NITROGEN CONTAINING COMPOUNDS 
Epoxy/amine cure materials 

URETHANES (ISO-Cyanates) 
Polyurethane Paint 
Urethane RTV 

UNSATURATED HYDROCARBON PLATIZERS 
HIGH ACID CONTENT PVC