Don't be upset at the number of views vs number of replies, most views will be by guests who are unable to reply.
Ok, the pics are pretty poor quality but from what I can make out, the welds look a little untidy, especially in the 4th and 5th pic. You don't seem to have been able to do a straight run, it looks in places like a series of blobs to fill a hole which suggests you weren't welding to particularly sound metal in the first place. It's far better to cut all the rot out so you only have a good, sound edge to weld to rather than a 'feathery' edge which just burns away as you attempt to weld to it.
There's a couple of holes in the last pic that need filling and the steel you've used is pretty rusty by the look of it which will cause problems. Even if you piant over the rusty surface it's still going to continue to corrode. Better to use clean, new steel to start with to maximise the chance of a good, sound repair.
Yes, the big holes, they look like they don't belong there but obviously they do .
A wire-wheel is ok but is no substitute for a grinding or cutting disc for removing rot. The pics would have looked better without the flash being on, most of the light is reflected off the welds. New steel will rust almost instantly if left outside, better to put some paint on it even if you have to remove it for finishing.
Keep at it, you're heading the right way .
If, after welding it, you won't be touching it again for a while then I'd say yes, get some sort of paint on it to protect it from the elements.
Looks plenty strong enough and once you've undersealed it you'll hardly see it.
Nissan Sylvia?? How bad is left chassis rail, just between drivers feet and engine bay??
preperation needs to be better,it looks like you went straight in with a cup brush and pulled bits of old underseal into the workpiece
get the underseal off at least 1 inch border away from the weld first by chipping it away with scraper or burning it off with hot gun
then wire brush in grinder, then a pass with a flap disc or some rough sand paper
the repair piece looks too rusty too, that should have been gleaming clean metal, better quality of weld and better earthing will be the result
good effort otherwise, wheelarch repairs are tricky as screwing yourself into a ball and straining your neck make it a lot harder
light is the biggest problem, i would have a 100 watt leadlamp wedged or clipped above the suspension