Fleur, You can always add layers as long as the piece your adding to is wet/dampen. Can't help you with bonding adhesive, I don't use. Billie

Fleur --- You can indeed add more layers. I do it routinely to work that was begun a year ago. As Billie noted, start by thoroughly wetting the work then allow the surface moisture to dry to the point that there is no "puddling". Then apply a bonding agent and your next layer...preferably with more bonding agent added to the mix. Do read the instructions on the bonding agent as I have seen some that state the new layer must be applied BEFORE the agent dries...and others that claim you can wait up to 24 hours (???). Me, I try to catch it while it's still "tacky". As a "PS"...the industrial spray bottles from the big-box stores are great for applying the bond...just make sure to clean off the nozzle before you set it aside for any length of time, as the bonding agent dries quickly to a film that will clog the sprayer. Go get'em.

I also use the wet it down method, let it dry a bit and add bonding agent when applying a new layer.
Tango, what a teriffic help to use a spray bottle.
I generally am using a small container and a cheap chip brush.
Fluer, You can use bonding agent full strength or diluted for the project you are describing above.
For projects where you are attaching one finished piece to another full strength has worked well for me.
Dena