Tuesday, April 15, 2014

Concrete Tentacle - First Cast

First cast of the concrete tentacle lamp base done. Not quite an epic fail but this one certainly won't see completion. The concrete is powdery at the top which is from too much de-moulding oil (vegetable oil). Thinking maybe I'll juts try using the PVA de-moulding agent for the next one. But worse than that, there's a crack at about a quarter of the way down which goes all the way round. It wiggles so I'm sure with a good pull, the tip will come right off. There is a PVC duct cast into the piece which ends right about where the crack is. I didn't bother trying to reinforce the concrete with wire but perhaps next time I will, particularly around the point where this crack has occurred.
And apart from all that, I used a six month old half empty bag of concrete that I found lying around under the house.

So next time, use fresh new concrete, use de-moulding agent sparingly and add a little reinforcing wire.

It popped out of the mould with no real trouble so it's looking good for multiple casts.


  1. Hi cam, I am making concrete planters and have had the same powdery effect. I was a very very thin layer of Vaseline in the mold but I'm thinking of switching to something else, what would you suggest?

  2. Hi Meina, I'm using vegetable oil. My friend who builds Skate Parks recommended it. I have tried vaseline but I think the vege oil works better. I've been wiping the mould with a paper towel soaked in oil, just to coat the mould lightly and evenely but not so much that the oil is "running off". lI let that sit for an hour or so, then coat it again.

    My understanding is that the oil stops the crystallization of the cement, stopping it bonding to the mould, which is what you want. But too much oil and you end up stopping the concrete setting properly so it ends up powdery.

    The other thing to watch out for is leakage. If the water is bleeding off it takes the cement with it leaving behind the sand and this may be another source of a poor finish.

    Hope that wasn't too much.


  3. I think the powdery concrete on this first one was because I'd literally poured oil all over the mould and let it run down inside so it pooled in the bottom. I'm being much more frugal with it now and the problem seems to have gone away.

  4. Wow thank you for your reply, have you ever painted anything concrete? I'm trying to spray paint my planters but some of the colours will literally be sucked up by the cement and the colour will turn white

  5. Hey Meina, I did paint these concrete pork chops


    if you can paint your planters with some kind of sealer first, your top coat should work just fine.

  6. Hi cam, thanks for that, do you wash the oil off in anyway or just let the concrete sit? How long did you let the concrete sit before painting? I have a feeling that's why my paint isn't staying on the concrete, maybe it hasn't dried properly

  7. Yeah, you'd probably want to give them a wash. I'd leave it at least a week. If it's spray paint out of a can then it's probably oil based so it really needs to be dry.

  8. Ok and would you wash the concrete after letting it sit for a week or wash it and then let it sit for a week? Sorry so many questions but no one else has answers haha

  9. wash it early, then leave it to dry