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I'd be using one of the Sikaflex products...


I've just used '512 to reseal some of the seams on the roof of our van...

..although with Sika' 512,I don't think you'll be taking the vent off again in a rush (lol) ...I dare say, there is probably a more "suitable" product in their range(..perhaps something which more for "bedding" rather than "sealing/sticking"..?)...but you won't go far wrong by using '512... ;-) )

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I bet the existing vent is sealed with soft mastic that doesn't set.

Whilst Sikkaflex is excellent ( I have just used it to seal the over door "drip strip" that was simply stuck on with some sort of sticky back spongy stuff) it will not bond to the soft mastic.


Removing all the old mastic is a right pain, so best to use a soft non setting mastic, and if you can match what's already on even better.


A hair dryer will be useful to soften the old mastic to get the old vent off or choose a sunny day, then use a length of fishing line to pull through the mastic, (like a cheese cutter).


Make 100% sure that you have a continuous bead of mastic of roughly the same thickness, the ready formed strips are good for this.



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Received wisdom (and the advice in John Wickersham's book "Build your own motorcaravan") is to use a non-setting mastic of the ribbon-sealant type to bed the rooflight on to the motorhome's roof, and then to run a tidy bead of an adhesive like Sikaflex-252 or Sikaflex-512 Caravan along the edge of the rooflight-to-roof joint for additional watertightness.


As has already been warned, if an elastic polyurethane adhesive like Sikaflex 252/512 is used to fully bond the rooflight to the roof, removing the rooflight subsequently will be a helluva job.


SIKA markets a non-setting sealant (Sikalastomer-710)




but I've no practical experience of using it.

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