Try Chassis in One from Buzzweld.
I have just done the seams under my sills and it goes on well by brush, dries quickly, seems very tough.
Can be painted direct to metal that has been brushed off.
Tip, use a round 3/4 inch brush to work it in well to any rough parts, (from any DIY store B and Q Dial make are not expensive. (ignore the sprayer in the photo)
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