I just posted a reply here making a recommendation for using Smooth-On's Rebound 25 brushable silicone and I wanted to post a little piece of advice for everyone, something I learned the hard way.
R 25, being silicone, doesn't usually need to be released before applying, it only sticks to itself- and shellac, apparently. By shellac I mean the amber-colored, bug spit, flakes-dissolved-in-alcohol type of shellac. We use it as a sealer all the time at work because it is ultra fast drying and waterproof (we do a lot of gypsum and concrete casting).
I used shellac to seal a cast iron piece prior to molding, to slick up the surface and as a precaution against inhibition of the silicone (the cast iron had been chemically stripped and platinum-cure silicone is finicky). When it came time to de-mold I found the silicone practically welded in place. It was tearing off. I've used R 25 for years and never had that happen before. Near panic ensued.
After lots of head scratching we ended up boiling the whole combo in a pot of water until the shellac melted and the rubber peeled off, lucky for us that the rubber (and the original casting) could take that kind of heat. Salvaged the mold.
Anyway, I hope some of you find this information useful. Be careful what you use to seal models with prior to casting!
R 25, being silicone, doesn't usually need to be released before applying, it only sticks to itself- and shellac, apparently. By shellac I mean the amber-colored, bug spit, flakes-dissolved-in-alcohol type of shellac. We use it as a sealer all the time at work because it is ultra fast drying and waterproof (we do a lot of gypsum and concrete casting).
I used shellac to seal a cast iron piece prior to molding, to slick up the surface and as a precaution against inhibition of the silicone (the cast iron had been chemically stripped and platinum-cure silicone is finicky). When it came time to de-mold I found the silicone practically welded in place. It was tearing off. I've used R 25 for years and never had that happen before. Near panic ensued.
After lots of head scratching we ended up boiling the whole combo in a pot of water until the shellac melted and the rubber peeled off, lucky for us that the rubber (and the original casting) could take that kind of heat. Salvaged the mold.
Anyway, I hope some of you find this information useful. Be careful what you use to seal models with prior to casting!
