Trailer welds,rust,& painting?

[RonnieT]

Have 1989 1900XR, has an escort trailer, no ID on the trailer but is the Maxum red trailer made from 2x4 Rectangle tubing.
Has 3 cross members, One at transom rear, one a 1/3rd of the way up and one about 2/3rds up.
The cross members have a VEE-type shape with a weld in the center where the angles meet.
The 2 front cross members have some rust-out, there in the bottom of this VEE and loss of some metal there about 2 inches at the most on each one.
I am a good mig/tig welder and could fix this with the boat on the trailer by protecting the fiberglass with a shield of Aluminum.
By chopping out the rusted VEE and making a new one of the same shape and material then migging it in clean and paint.
As I was under the trailer inspecting this I see that NONE of the Tubing joints are welded underneath, even at the trailer tounge area. All tight-fitting but no welds on the bottom of joints .
Did they do this on purpose maybe to drain out water?
Or could they not weld overhead? Maybe didn't have a rotator to turn trailer over when manufacturing.
Do you think I should weld them up and paint them or leave them alone?
Just wondering and would like your members opinions,
RT