Ban all fillers
At Fellert we took a decision several years ago to ban all use of fillers when installing our system. At about the same time we invented a special clip for mechanical fastening of our absorbent boards to suspension grids and thereby also banned the use of washers (discs).
The reason for this is quite logical. In order to be a sound absorbing surface it needs to be porous, where the sound energy will find the pores and while using them to penetrate the material its energy will be absorbed by the “walls” in each pore, like this:
In addition to this you will also have a “micro climate” with air flowing through the absorbent board. This works as a chimney where there is a difference in temperature below and above the absorbent board. This thermic difference will start the air to flow from the warmer area towards the colder area. This airflow will bring with it, airborne particles such as dust, sooth etc. and these particles will attach to the surface and some of them will also penetrate the pores of the porous absorber.
If you are using a filler to fill up the joints of the absorbent boards, this filler will not have the same porosity as the rest of the absorbent. Most likely it is of higher density and less porous than the surrounding material. The air flowing through the absorbent will not be able to flow through the filler the same way as it will through the surrounding material. Less air will flow through the surface with a filler backing, which in turn means that less dust and sooth will get attached to the surface and penetrate the pores.
The result will be white grid marks and dots in your seamless acoustical ceiling.
This can easily be prevented by banning use of fillers and disc washers, which we did several years ago.
Take a look a the step by step installation guide to get more information on how to install Fellert. You’ll find it at http://www.fellert.com/facts-of-fellert/instructions/