Which Sealant

I used Sikaflex 512/522 when I replaced a high level brake light

Do a full check on SIKAFLEX variants as ED says  --  Get the best one for the job, cos its good stuff  

I would also use the sikaflex primer if recommended for the surfaces you are sealing. I tested with and without the primer prior to a  repair a few years back and definitely recommend it. 

My joint prep whether bonding two parts or 'tidying' up the sealant on awning rails or roof strips is:

Applied brake cleaning solvent using a sponge swab to clean/dewax the area to be bonded, assuming you have removed old/defective sealant.

Swab repair area with  sikaflex primer with sponge swabs

Wipe swabbed area with clean non coloured paper towel and apply sealant ASAP

I have also had a problem with leaks in torrential rain when water puddles on the roof. I think mine are caused by the body flexing on hitting really bad bumps in the road which breaks the seals. I have resorted to using a marine version of Sikaflex. It is reputed to be a tougher version of 512.. Its 219i and I got it on the advice of a marine chandlery store but you should be able to  buy it on line. I thought it worth a try.

peedee

I will be resealing my Bailey Ranger soon, it has leaks on roof and awning rails. As Watto64 says, surface prep is very important (I used to work on aircraft sealant testing). White Spirit should work to remove sealant, but it does contain some oil, so that brake cleaner is needed before applying primer.
The other problem I have is cracked mouldings, possibly due to rust-expanded screws. As an emergency repair I applied hot melt glue from a Bostik gluegun, it worked short term but the glue isnt UV resistant enough. The mouldings seem to be vacuum formed ABS or something similar, so I bought a cheap 'plastic bumper repair kit' online and will try that before resealing the mouldings to the cover strips.

I've used Soudal RV61 SMX Polymer sealant and adhesive. Suitable for interior and exterior, and was recommended by a dealer. Its done the trick

I've used this product recently , it's extremely good with High grab hold properties.

It was used internally to fix some seating furniture framework over the O/S wheel arch that had moved after going over speed humps , its done a brilliant job.

As stated it's sealant & adhesive in one tube , readily available from Screwfix.