I agonized over this as most modern cars have a skirt on the underside of the wheel arches to stop any stones flying up and hitting the vulnerable parts. I did consider putting in a skirt this but decided it was too much kerfuffle, so I decided to just slap on more underseal and not go on any newly dressed roads.
I used a 4.5 ltr tin of ISOFLEX and made sure I had my oldest clothes on - and gloves. The ISOFLEX needed to be applied in stages; each stage [coat] being about 24 hours apart. So that stitched me up my next couple of days for doing other things. No matter, though, it's a one time deal and the sooner done the better; then I will be able to fit the lights once and for all (famous last words.)
Three days later: three coats applied under the wheel arches, two in the nose cone and engine area, and I used up the remainder on the back axle cross tunnel.
Job well done and what a thankless job this was. I never want to do this again. But I will have to do the flat middle underside bit of the car when I fit the body on the chassis. Can't get to it now as the car is on the trolley so that will have to wait. But... this underseal job will be a pleasurable job as it will be one of the final jobs before the big day.