Prepping:  Molds should be clean and ready for release agent.  The best over the counter release agent for aluminum, latex and plastic molds is caster oil and 70% alcohol that can be found at any grocery store or Wal-Mart store.  Ratio is roughly 2 oz. of caster oil to 16 oz. of alcohol, placed in a spray bottle.  This mixture needs to be shaken repeatedly, to keep oil and alcohol mixed.  Sprayed in a fine mist over entire pouring surface of mold.  If sprayed too heavily it can affect the surface of the casting. 

     There are superior products on the market for releasing molds from aluminum, latex and plastic.  Make sure you have the right product for the right type of mold being used.  Ask questions if the release agent will work properly on aluminum, latex, silicone, urethanes and plastic molds.  These products help protect the mold, release better and keep the mold far cleaner.  In my opinion well worth the cost. 

     If you have purchased plastic that is too light for the volume of concrete being poured into it, bulging or collapsing of sides; building a simple wooden frame/box to keep sides from collapsing or bulging in undesirable areas should help take care of this problem.  Heavier plastic, such as 1/8 ABS used in my bench tops to 3/16 ABS used in my bench legs, should not need any support.  A Polyethylene plastic may be thick but does not support concrete well and will need a frame.  You may want to ask the seller before purchasing the mold and causing more headaches and work for yourself. 

REPAIRING LATEX MOLDS:  A quick fix you may use is superglue to close rip.  Then a tire patch on the back side for strength.   

Cement Mixtures:  I'm going to keep it simple on mixtures.  I use portland cement, mixing sand and pea gravel to the consistency of a pancake batter.  1 part portland cement, 1 part sand, 2 parts pea gravel.  Pea gravel will add strength and may be used more with statues.  A little less gravel and a little more sand may be used for smaller detailed pieces such as 1 part portland cement to 1 1/2 part sand and 1 1/2 part pea gravel.    If your mixture is too dry it may be hard to get air bubbles out.  Too much water your item can become sandy or worse. 

Ready mix concrete, sand gravel and concrete premixed in a bag, not my choice but will work.  When using ready mix I generally keep a bag of portland concrete to add a little to the ready mix.  The problem being with ready mix is the mixture may not be consistent from bag to bag.  Problems usually show up when conditions are cooler, more humidity or general atmospheric changes while concrete is setting.

Coloring:  White portland concrete can be purchased sometimes through Home Depot, Lowes or pool companies (specialty stores).  Which will leave your finished product white throughout.  I prefer to add colorants to my mixture so that the color is throughout the casting.  These can be purchased in liquid form (buff, terracotta, brown, grey) from Lowes and Home Depot.

Acid Stains:  Generally have to be purchased at specialty stores.  These are brushed or sprayed on after the casting has been removed from the mold and let dried.  Downside, if chipped the coloring is not through and though.

Pouring Cement in Molds:  When pouring into a plastic mold you want to cover the bottom with a thin layer of cement no more than 1/2 inch of concrete, just enough to cover the bottom.  Then tapping on the bottom of the mold to release air bubbles.  If you pour too much cement it is harder for the air bubbles to be released from your finished surface.  This is also true with most latex and aluminum molds.  Once you have done this you can add the concrete more generously to finish filling the mold.  Larger items like bench tops you may try sliding the mold to the edge of the table and tapping gently on the bottom to release air bubbles, doing one side at a time.  Drying or setting time is generally 24 to 48 hours, depending on weather conditions.  I recommend on plastic molds NOT filling to the very top of the mold, leave roughly a 1/4 inch or so from the top so when you are ready to unmold it will have space to release.  This may not be possible on thinner molds.  Vibrating causes your solids to compact to the bottom while causing the water to rise to the top of the mold.  Making my own vibrating table by attached a vibrating sander, even a reciprocating saw, to a table protecting the electrical parts from water.   I've also held a palm sander against aluminum and latex/fiberglass molds to release air bubbles.  Be careful, electricity and water do not go well together, can be extremely dangerous. 

     Reinforcement rebar is generally used in all bench tops.  I generally use reinforcement wire from a roll of reinforcement mesh used in cement to give strength to my smaller crosses and 1/4" rebar in my larger crosses.  I like using 3/8" rebar in my bench tops, this allows me to use large bolt cutters to cut my pieces. 

     There are several types of adhesives for cement which can be used to attach items such as bench top to legs.  You may want to consult your hardware store for the right product for your application.       

     These are just helpful hints and suggestions to help you get started and by no means will I be held accountable for the intended help and suggested hints.

     Good Luck and Thank You, jw12754.