Yes I basically light a candlestick and hold it over the flange joint to let the wax melt into the gap between the flange and the part. To make it easier I turn the dash and rest it at various angles so I can easily get the wax to drip in the right place. While the wax hasn't fully hardened I scrape off the excess with a plastic knife or wooden lollipop stick thing (actually its a doctor's tongue depressor). This helps give a nice slightly concave edge.
The hot matt doesn't seem to melt with the wax as this is the process I used last time. Also remember that the Gel Coat goes down first and that isn't that thick and doesn't generate that much heat. Once that its semi-hardened it doesn't really matter how much heat is generated with the thicker layers of matt as the wax is already sealed in with the Gel Coat. Then when I put the mold parts back together again I use the same process and fill the mold gaps with candle wax also to minimise Gel Coat flashing lines.
I got this tip from Brett on the Sterling site, he does some great work. Worth a look at his thread.