Finally managed to get the old ply floor out of the van. Found the rear wheel arches "floating" above the floor as joints between bottom of the arches and floor have rusted away, so will need lots of welding
Once all the welding is done I plan to tape over the original ply floor self tapping screw holes with aluminium tape, spray the whole floor with either waxoyl or underseal (cans), lay down 3mm laminate floor underlay then re ply and paint with yacht varnish ( should I paint the underside of the ply as well?). eventually will have the whole underside of the van sprayed with waxoyl.
Does this seem like a good plan? suggestions welcomed.
Cheers
Colin.

