Contact cement is a uniquely useful adhesive. It will adhere to most all surfaces and hence is often the in-between for sticking two different materials together.
It comes in two basic varieties, solvent based and water based, the later often being called "green" or "solvent free". Although these two are generally inter-changeable there are some important differences.
DIFFERENCES BETWEEN WATER AND SOLVENT BASED CONTACT CEMENTS
Water based contact cement gives off no VOCs. Solvent based contact cement should not be used on foam insulation as the solvents will eat the foam.
Water based contact does not stick well to metal or glass but solvent based does. Neither sticks well to masonry but interesting enough they will stick to each other. For a chart of all of this check out WHAT GLUE WILL STICK TO WHAT?.
Water based contact cement is more expensive per ounce or ml than solvent based contact cement but because it spreads further, it is less expensive per square inch.
Water based contact cement actually has a higher bonding strength and can withstand higher temperatures than solvent based contact cement.
Both can be reactivated by heat -- something that is useful if you want to shift or replace something like a plastic laminate -- simply warm it up with a hot iron, cool enough to not bother the laminate but hot enough to soften the cement. This is also something that can be a problem because objects attached with contact cement can loose their grip when they sit in the sun for a while -- hence it is rarely used outdoors despite its waterproof nature. My personal crisis with contact cement and the sun came with a brass plaque that I attached to a piece of plywood screwed to the wall of a meeting room. When the sun rolled around to shine through the window onto the plaque, it came crashing down right in the middle of the meeting that it was installed for.
Contact cement is applied in a thin even coat to both pieces to be joined. The joint should be good fitting as the glue line is not thick enough to fill a gap.
If the humidity is over 80% water based contact cement may not cure at all. Depending on the temperature and humidity the surface will become dry but tacky in about 15 minutes. When the glue will not come off on your finger, but it has a tacky feeling, press the two pieces together. Press tightly and it is done -- rolling over flexible materials is a good idea. Clamping is generally not necessary.
On very porous surfaces, like the edge of plywood or pressboard, you will notice that the first layer of cement simply disappears into the wood. I always put a primer coat of the same contact cement on these edges first and let them dry a couple of hours. Then I apply to both surfaces -- now my wood side adhesive no longer just disappears into the wood, in fact the first coat gives it a good grip. If I am applying heat applied banding to the edge of plywood or pressboard (that banding heat sensitive adhesive is very similar to contact cement) I first put on a primer coat of contact cement and let it dry thoroughly. Then I heat apply the tape and the two fuse. Priming the porous surface means that the tape will never come off.
THINNING CONTACT CEMENT
Both water based and solvent based contact cements can be thinned but always with a loss of adhesive properties. Generally if you find solvent based contact cement too thick, change for a water based, which is already quite fluid. If you really want to thin this stuff, use water for water based cement and Acetone or d-limonene based glue remover (as indicated on the can for clean-up) for solvent based contact cement. I have heard of success using roller and brush cleaner containing Toluene and Acetone. Any other solvent, like lacquer thinners and gasoline, will most likely attack the basic components of the cement, doing more long term harm than good. Contact cement, thickened by age, cannot be thinned -- the bonds have already started to form and will not reform if broken down by the fresh solvent.