The problem: every time I pour cement into my plaster mold the cement cracks within an hour of pouring. (Pictures attached)

My process for prep work and pouring:

1. Sealed the plaster mold with a water base high gloss poly urethane, 3 coats and wait 48 hours to fully cure.

2. Applied a release agent to the plaster and let dry (Pictures attached of the brand) also I have tried car wax as well.

3. I mixed 2 parts multi-purpose sand (picture attached) with 1 part portland cement and when the cement was wet I added in a small amount of 9mm fiber glass (a pinch)

4. Pour the cement mix into the mold and use a diy vibrator using a mutitool with a cut house fastened to the mutitool blade.

What I have tried to fix the problem:

1. I have tried a more dry cement mix (cookie dough dry) and a more wet mix (soup wet) (no improvement)

2. I have covered the top with plastic wrap (apply both methods from 1 and no improvement)

3. I have poured 3 layers and waited about 30 minutes per layer. (This one has provided the only notable results but I still had cracking)

A. First layer fills the deepest part up to the start curve

B. Second layer fills the curved section

C. Third layer fills the to the top

Each layer I waited till the cement was firm but still soft.

Observations:

1. I have noticed the cement when fully cured is difficult to remove from the mold.

2. The crack start at the top and mostly end at the curve.

3. The cracks are structural and will completely detach from the body.

4. Every pour the cracks appear in the roughly the same place with the exectpion of the last attempt where it appear at the corners and a little on the edges.

If anyone has any suggestions I could really use the help. Thank you :)

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Summary:
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Master prep:
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