Hardwood Flooring Trowels - Specs
Unfortunately what many wanna be installers fail to 
recognize is what type of trowel should be used. All products are different. One type of installation may require a different sized trowel than another. Wrong sized trowels used are one leading contributor to failure with glue down floors.
Manufacturers Specifications
When buying hardwood floor adhesive it is vital to look at the manufacturers specifications. While some manufacturers make their own proprietary glues and others recommend Bostiks, Sika, Mapei, and DriTac to mention a few, none of them have the exact same specs.
In addition to the proper trowel requirement, too little or too much adhesive can affect the installation. For instance a heavy amount of adhesive spread can create fitting problems if you're not experienced using the stuff. Essentially what happens is glue is scooped or lodged between boards when trying to push boards into place.
Considering the properties of some adhesives, cleanup is sometimes not an easy chore. Using less than the recommended amount of adhesive (often directly related to the trowel notch size) can create voids between the subfloor and new floor. Hollow spots may prevail or popping sounds, and squeaks at a later date are likely to occur.
Make Cleaning Trowels Easier
I don’t know why I never thought of this idea when I 
was actively installing floors. When urethane adhesives cure they’re tough as nails. Grab some duct tape and tape the entire trowel except for the notched areas. That way all one has to do is rip off the duct tape the next day the trowel is used. The old way was a royal pain by scraping the adhesive off.
Multi Use Adhesives
Over the last few years more multiuse adhesives have been gaining acceptance. Most manufacturers have their own proprietary clip on trowel adapter that must be used for the work.
From Our Readers:
Subject: Glue Slumps After Spreading
Question: I have been installing some floors with the Bostiks glue recommended and wondered; it calls for a 3/16" thick V notched trowel but after it's spread and sits for awhile the stuff flattens out. Do I have some defective glue?
Reply: No it's okay. It does however, drive some installers bananas. The fear is not enough adhesive transfer that may not provide a good bond between the sub floor and wood. Making sure enough transfer taking place starts with applying enough pressure on all boards equally. You can easily see that by testing.
