by John61CT » Fri Feb 09, 2018 12:22 am
Glue rigid panels up, then fill the gaps, finish with light fast passes to build up a bit more depth.
If you're really OCD, include small battens and glue a final 1/2" inner panel with holes every 4", and poke the spray in with a little spritz in each.
Have flat ply, sprayed with release or covered in plastic and poles ready to hold up with a little pressure as you go, but only put them up just after any (ideally very little) excess comes out the holes.
Otherwise a little irregular air gap does no harm.
I agree sawing large areas parallel to the surface is a major PITA and waste of material.