We just bought our first rot-box on wheels. Having read all the sites I can find about how to go about repairing it I'm stuck with applying the mastic to the awning rail (on the non-awning side). Our van is a Swift Pirouette, about 1992 vintage and has rails with a deep channel in the back (about 4mm wide x 4mm deep) that the fixing screws come in through. This was full of old mastic, about the only stuff that wasn't hard and crumbly. There are black rubber(?) trims along each edge of the aluminium rail that clip into further grooves along the back of the rail, these had contracted back away from the joint in the rail leaving a 3cm gap.
What's the best way to put mastic on these rails?
I've got some 4W mastic strip that is 32mm wide and a tube of sikaflex. The width of the rail with the rubber seals fitted is 38mm.
I would fill the channel with tube mastic , sikaflex 512 will do the job , but is expensive , seeing as it's the non awning side you could use a cheaper caravan tube mastic . The mastic has in my opinion got to do two things , 1 seal ( stop water entering via screw holes or joins ), 2 help hold the rail on the caravan ( more important for the awning rail side as the awning when up, places quite a lot of load on the rail .
Remember to add a bit of sealant to the end of each screw before fitting .
Does it make sense to then use the mastic strip on the back of the channel with a bead of 512 along the rubbers to make a neater job?
You could do , the problem is though , having the black rubbers on the rail . With these on there i'd say you would probably be better off just filling the channel providing it seals the screw holes and any joins . If you were to add the roll mastic it will make the rail further off the caravan body , which is unnecessary in my opinon . Other sealants wise i'd use either Tiger_seal or Soudal Fix_all or Soudal Carbond 940 FC.
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Topic: Grooved awning rails
. Having read all the sites I can find about how to go about repairing it I'm stuck with applying the mastic to the awning rail (on the non-awning side)
. Our van is a Swift Pirouette, about 1992 vintage and has rails with a deep channel in the back (about 4mm wide x 4mm deep) that the fixing screws come in through. This was full of old mastic, about the only stuff that wasn't hard and crumbly. There are black rubber(?) trims along each edge of the aluminium rail that clip into further grooves along the back of the rail, these had contracted back away from the joint in the rail leaving a 3cm gap. 
