Upol is okay. I use Screwfix gal spray. None are good to weld through, just put them on areas you can't get to with paint later. It is only a single pack primer, so you should be looking to overcoat it with a wax treatment etc.
Weld through primer is just for the places you cant get to after welding such flanges that have been spot or plug welded together. Any weld through that is visible afterwards should be cleaned back to bare metal ready for something like epoxy primer.
For areas like the inside of a box section you can prime with epoxy up to the edge of the flange then put weld through on the flange. Any over spray onto the epoxy does not matter. As Burdekin says, then put lots of cavity wax in the box section.
You could try some test pieces partly primed with epoxy and see how close the weld can get to it before the paint bubbles. You don't want bubbles as they represent unpainted metal that the wax can't get to.
I wouldn't do that as it could easily block up the drain holes and channels which are quite narrow, with wax you can easily clear with a wire afterwards.
havent tried injecting epoxy,but think it maybe hard to get into all the seams properly. when i fitted a new crossmember with extensions to my defender i flattened the black cathodic primer down with a scotchbrite on the underside ,sanded down the out outside with 240 grit, gave the whole thing 3 full coats of davids zinc rich primer ,painted all surfaces with some white 2k that was lying about, cleaned up the lugs that had to be welded,alinged everything up 100% ,welded up all sides ,brushed zinc onto the welds, then when the visible side had been painted with black coated the underside with black waxoyl, my theory being i knew it was properly covered when the white paint didnt shine through,if i had simply waxoyled onto the black cathodic i could,ve easily missed a bit (did the same with the new bulkhead outriggers).i treated the inside of the xmember to ordinary wayoyl with a high pressure gun til it started to run out. I since sold the defender, it still is in my area and saw it not long ago, all replacement chassis parts are still pretty clean and solid as a rock, my point being a little bit extra work pays off in the long run especially with restoration.