Is it safe to use plastic on inside of concrete fo

Thanks for any help!

I can suggest using a concrete vibrator, but make sure you are heavily braced ( depends on height ), use the correct amount of tie backs or whatever configuration you have to make the form strong, no overkill factor here few extra braces, tiebacks won't hurt. You want to use the concrete vibrator to settle the concrete, but not over do it, just hit an area once, quickly and move to the next. Once the first foot or 2 is in the form, run through and vibrate it, then place another foot or so, repeat, don't wait until the form is full and then plunge the vibrator down in, you will find the weak form as a lot of concrete will settle at once and create quite a bit of pressure against the formwork. Had a blowout on a form from this once, person directed me to stop and then use it after the form was full, yep, shovel it all out, fix form, re-place concrete, vibrate again carefully was not noticeable at all, but who needs the extra work, stone was covering the finish anyways, vibrating or even using a rod to pack and settle will help the finish and reduce voids on the surface. Smaller form, a few whacks on the side may help settle, I like the concrete vibrator the best, but use it cautiously, it works very quickly in a small form.

I agree the OSB board works fine, also you can get and use a pump spray container with the wand, to spray form release agent onto the face of the form, this helps avoid stuck form work, another thing I do when I don't have release agent is just soak the form with water, doing that does not allow the water in the concrete to be absorbed by the dry form or dry ground, seems to work well to keep things from sticking, OSB would be better to use release agent from your masonry supply house, soak it too much and it may swell, but wetting it down just before the pour may be all you need. Just a few thoughts here.

Even 6 mil plastic sheeting will work, but then you get Sid's wrinkle possibility if you don't get it stretched. Plus, plywood knots/grain will easily telegraph through. Does give an interesting texture.

I've place all sorts of texture material inside concrete forms. The concrete never complained.

Have fun.