My platinum silicone came out tacky / gooey / won't fully cure. What happened?
This is almost always cure inhibition: platinum (addition-cure) silicones are poisoned by contact with certain materials, leaving a sticky, uncured surface against that material. Tin-cure silicones are far less prone to it.
Common inhibitors, if your silicone touched any of these, that's the cause:
- Sulfur-based modeling clays (most non-Sulfur-free oil clays)
- Latex, and many urethanes/epoxies that aren't fully cured
- Tin-cure silicone residue, and some adhesives/tapes
- Certain 3D-printed resins and PLA/PETG, a very frequent modern cause
- Condensation/moisture or recently used cleaning solvents
The fix, always test first: Before a full pour onto any new surface, do a small test patch. If it cures clean, you're good. If it stays tacky, seal the surface first: a barrier coat (e.g. a compatible sealer or a thin acrylic/shellac layer, fully dried) blocks the inhibitor. For 3D prints, ensure they are fully washed and UV-post-cured, then seal before molding.
To rescue a part that's tacky only on the surface: remove the silicone, clean the master, seal it, and re-pour. Inhibited silicone cannot be "re-cured", it has to be redone on a sealed surface.
Learning Center
Full guide: Curing Issues · Diagnosing Potential Causes