Sills protection

Im going to fishoiline my beetles sills but there seems to be some debate on how you access them. Some say you have to bore holes, insert the stuff then weld them back up again ..... doesnt appeal as the car is pretty time warp as new now.
Some say you can poke the hose in through the front floor well heating vent (after removing it) or remove the curved part of the heating channel under the rear seat. and poke the hose in through there. ..... Sounds good BUT will that get the stuff to the full width of the sill? Peering into the channel, having removed the front floor well vent, I detect that the heating channel is only half the width of the total sill ..... but maybe its only for appearences around the vent and opens up once out of sight.

Does anyone have knowledge of the sills ...... will feeding the stuff in through the heating vent holes do the job?
She is a beaut bug, but shes 39, and I have seen some terribly rusted out sills over the years .... on cars that didnt look too bad otherwise but where the rot had started from inside, probably not helped by condensation from the heater. So want to protect mine from inside.
Your knowledge would be greatly appreciated.

If the sills are original, the heaters are ducted in their own tube, so you'll need to rust proof both inside the heater tube and in the actual sill by drilling access and sealing the hole with a grommet. If drilling into the bottom panel, keep the access hole closer to the running board ie the outside of the panel, I normally put two holes in, one under the A pillar and one about 500mm fwd of the back of the sill.