What area are you painting? :?
Anyway, weldon does not work thru the paint, so you must either cement the part together first, or, if you want to do more work and get the same result, you can do either of the two below:
1, paint the parts, then plane/sand/strip the paint off the area that need to be cemented, and make sure you do it in such a way that it won't craze when you apply the solvent.
2, mask off the area to be cemented, paint the part, than cement the parts. Tricky think is that you need to make sure: a, the masked area matches. b, the paint does not get in the way of the cement later.
In other words, cement before paint unless you can't help it.