I've been using acetone for decades. As the acetone evaporates the epoxy will get thicker until it cures hard. It works very well for thin layers for fuel proofing the firewall and fuel compartment.
When applying glass to a fuselage or any other part, wet (saturate) the cloth with resin on a piece of clean cardboard, then apply the whetted cloth to the part. You can then use a squeegee, roller or stiff bristle brush using a stippling motion to remove the air bubbles, smooth the cloth out and remove excess resin.