How similar to RTV silicon is Selley's gap-seal silicon?

If it's a solvent-based (one-part) silicone, it will shrink as it hardens, whereas two-part, catalyst type RTV does not. Most tube silicones are also adhesives, so a master part will have to be generously coated in mold release or it will be permanently bonded in the silicone when it hardens.

Gerald Owens

I know about the shrinkage part - had to redo around the shower because of it.

That satisfies my curiosity.

While others have pointed out the shrinkage and adhesion problems, I do remember seeing, several years ago, an article in FSM about duplicating very small parts using the tube stuff to make the molds.

Does silicone have solvents? I thought it was a hydrolysis thing, where the rubber is somehow held in solution by being an acetate, but exposure to water in the air makes the acetate come off (thus the vinegar stink) and the exposed sites on the rubber polymerizes. I'm pretty fuzzy on the details, obviously.

You are essentially correct. The one part hardware store silicone sealant isn't actually disolved in acetic acid. It cures by absorbing moisture out of the air which allows it to polymerize. Acetic acid vapor is (smelly) a BYPRODUCT of the reaction.

Greg

Since we're a bit off topic anyway....as an aside, DO NOT use run-of-the-mill tube silicone around showers, windows, doors, etc where there is contact with mortar. The acetic acid reacts with the lime in the mortar and deteriorates it, making it crumble fairly quickly. Use only non-acid silicone for these applications. Even the paintable caulk "with silicone" is acid based,so beware.