Molding & Casting Materials

im sure im in the minority as well. i have never used a mold release. i know, i know, BAD. but i have never had any issues getting my casts out of my silicone molds and the molds are holding up really well. so i just just wash the casts with some soap and water in the tub after i dremmel em out. then i use a good primer. either a plastic specific or an auto primer. i don't go cheap there. i've had too many nice paint jobs flake off because i've gone cheap primer in the past.
and that's it for me.
 
I do not know a lot of people who use a mold release on silicone. Half the reason to use silicone is it is kinda its own mold release. If you use a good quality mold silicone, the mold will last for many runs.
 
I was planning to use a mold release (to be on the safe side) but it does seem to cause more problems than anything. How much life are you getting out of a mold Daman, 3 years?
 
I've stop using spray mold release for regular castings. I just haven't needed it. I use the mold release when making the mold only to keep the silicone from bonding to the silicone. And very sparingly on the actual master. The mold release will show up in the casting. I use talc for the regular castings.

Although, I'd say that the more rough the surface the more I would consider using the spray. I've got a few dozen castings out of my first blade mold with several different types of resins/plastics with no noticeable degradation in the mold and I've been using talc for most of them.
 
well i havent had a lot of castings out of any particular mold. one of my firsts molds. i used caulking silicone and it had a lot of small porous details, and it is still in fantastic shape. that one has had the most pulls with about 15.
i have never used talc or any powder at all
 
Both Smooth-On and Polytek have told me to use talc. This is mainly for the surface tension and to help reduce surface bubbles. That's pretty important in my narrow molds. I can't see me not using talc.
 
Okay, should be getting around to some molding soon. Curious, the only day with a decent temp this week will be rainy. I'll be in my workshop with windows/doors open and fans for exhaust. Being indoors is there any chance of the rain still effecting the fiberglass/bondo shell as it cures?
 
polyester can be a tad sensitive to moisture. Regular gel coat sometimes wrinkles in heavy humidity. Bondo resin mix does not, usually. You can always run a dehumidifier if you are worried.
 
Both the fiberglass resin and the bondo body filler are polyester based. Should I continue on the day I planned or should I wait for better weather? Any and all are welcome to comment. I need a few opinions.
 
They are indeed both polyester based. However, the body filler contains talc and that helps. Normally if you add to much catalyst to polyester, it gets hot and cracks. The talc buffers some of this, and also helps with the moisture. That said, not knowing the exact conditions, temp and humidity levels of the room, it is hard to guarantee anything. I have glassed on rainy days in LA, and really humid days in China, like 70% humidity, sometimes a bit more. If it is warm in the room, that helps for sure.
 
What kind of humidity percentage will I need to look out for when casting my resin pulls or is that an issue? Also, are polyester brushes okay for silicone?
 
try to keep humidity below 35% and keep room temperature at about 75 degrees F, or a bit higher. With lower temp and higher humidity, you might need some extra catalyst. Always use clean brushes. I never mix brushes, so if you like to rinse polyester brushes with acetone to reuse them, only do it with polyester. Use a fresh brush each time with silicone.
 
I've never had issues with humidity with any materials other than cold foam, and that was down in south east texas, where the humidity would get stupid sometimes. I wouldn't worry about it.
 
Polyester is sensitive to humidity. We get 70-99% here in Guangzhou, my resin has warped while curing because of it. Solution was a dehumidifier and more catalyst.
 
Interesting, I never had that problem, and it was a rare day when the humidity was below 55%, usually more like 60%. Still, can't hurt to take precautions.
 
Yeah, the weathers a little odd around here. One day the humidity could be in the high 70s and the next high 30s. I just don't want to fudge up anything I'm working on due to it. As for the polyester brushes, I don't plan to reuse them. I was just concerned about them affect silicone in some odd way while making a mold.
 
I guess I understood your meaning of polyester brushes. You mean polyester bristle fiber. I would use natural bristle brushes to be safe. Some silicones are sensitive.
 
Okay, so out of nowhere the weather changed on me and went from 75 to 65 in less than an hour. Bad part is my rondo shell was just finished. It's been an hour and it's yet to be fully cured. No heat lamp, no room temp control. Whats the best way to cure this thing? I was thinking about standing over it with a hair dryer. Could I also do the fiberglassing this way? Need quick advice.
 
Even in 65 degrees it should have cured. Bondo resin mix is pretty tough, much more so that standard polyester. Are you sure you used enough catalyst? Hair dryer is probably your only option now. If it does not cure after 30 minutes with a hair dryer you might be stuck. You are going to have to add more catalyst to the polyester when you do the fiberglass because of the temperature drop.
 
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