Radio Code Guru
The reason I said to do it that particular way Paul is that the act of welding the end panels on will just heat up the sill and burn off all of the Waxoil in that area. Far better to spray the completed,sealed cavity, using a perforated 'lance' (a length of 6 or 8mm nylon tube, long enough to reach the whole length of the sill, with the end blocked but the last 6" pierced with multiple holes, around the tube so that when it has the waxoil pressurised into it it sprays out for 360 degrees and as you withdraw the 'lance you coat the whole length of the sill cavity), inserted through a largish hole in the rear end, which you can inspect through to check that you have a good coverage the whole length.Good idea john.
Actually while it's open for easy access, I'll grab some waxoil n spray right down the sills before closing it up. If I need access in future, I'll drill a hole when required.