It comes down to if you think you might have to make a repair later. If you put the J rail on after painting, then you can more easily reseal the J-rail. Also, if you have a fabric cover that will be going over the trailer, then the fabric of the cover will over time start to take away the paint. Wind will billow the fabric and move it around. Then you will start to see the outer edge of the J rail start to show through, eventually.
Some people just like the cleaner look of painting the J rail after it's up.
Personally, I will paint first and then mount it later, just in case I end up with a leak at some point. At least that's how I understand this.
See who else posts. It's been really slow around here as summer months get really slow on the board.
"I get that queasy how in the hell will this thing ever go back together feeling.” - PT
No paint on the Jrail or Windows... All trim should be free of paint to really pop... But I have done trailers with painted J Rail also. And eventually the paint will chip off here and there and need to be touched up. Your call.
God grant me the strength to restore the trailers I can, The courage to strip the parts off the ones I can't, And the wisdom to know the difference...