H,

 

Longstanding Octavia owner, having recently moved from 06 to 08 model.

 

One of the issues I hade with the 06 was both cills rusting from the inside to outside. The cills on the 'new' car are in fine condition, but as I tend to keep cars for a long time, was looking to 'waxoil' them or sothing similar. 

 

Are there existing maintenance/drain holes in the cills that I could access to spray the inside, or do I need to drill and plug? If drill and plug is the way to go, where best to drill?

I don't really know the answer. Without knowing the exact construction under the cills it will be test drill small pilots and then enlarge. Probably along centre line of top flat surface. You could try to remove door seals and see if there is any access pre existing but I doubt it.  

Drilling three holes about 7 or 8mm in diameter will allow you to get a waxoil flexi lance into the sill and then poke the lance around reaching all the areas while blasting the gunk at some increased pressure.

Drill them centre and about 150mm from each end, at the lowest edge of the sill. Dont forget to rust-proof the holes properly (seperate issue altogether but epoxy resin, albeit a bit expensive for these small areas is ideal) before plugging with some simple rubber grommets to stop water ingress once the fluid is injected to your satisfaction.  Good idea to plug the two other holes while injecting into one also so that you get a good flood into all areas.

Take a look at rust.co.uk as they have a new product that allows you to flush cavities out prior to treatment that primes/treats any existing rust.  

I now a thing or two about how to fend off rust as i own a 1962 VW Karmann Ghia AND converting a 12year old VW Crafter into a campervan.