Hi,
I´d say it´s a myth that adding solder can significantly increase current capability.
Now what is "significantly"? 50% more current? twice the current (100% more)? 3 times? even more?
The shown PCB looks like a cheap product from the 1980ies. At this time they mainly used solder with about 35% Pb.
This solder has about 10% (9..13%) the conductance of copper.
So if you want to reduce the resistanc to - let´s say - half of the initial value than your solder (including Cu) needs to be about 10 times the copper thickness. So from 35um to 350um. And it needs to be as uniform as possible, because the parts with lower thickness will have the highest impact on overall resistance.
On the shown picture the effective solder thickness is less than 350um. And - even worse - it´s not uniform across all over the copper trace width. It rather seems to cover maybe 70% of the width.
So I expect the resulting resistance maybe is 60-70% of the initial value. Or 140 - 160% of increase in conductivity.
But this does not mean you can run 140..160% of the current, because P ~ I * I
so yu have to take the sqaure root. The increase in current capability is 20..25%.
***
Out of curiosity I did an experiment.
I had an old PCB with a 35mm long, 1.27mm wide, 35um thick trace.
Math said it should be about 14.5 mOhms.
I measured (all very accurate with 4 wire and at identical temperature) 13.4 mOhms
(This is less than expected. Maybe because of my messurement errors in length and width. .. or a bit higher copper plating)
Then I added solder over the whole width with "usual" thickness. Maybe 0.2mm effective:
Result: 10.8 mOhms (identical to increase in width from 1.27mm to 1.57mm .. just 0.30mm)
Then I spend some time to put some ugly thick solder on it. It really was hard to do this. And it looked awful. I guess there were parts with 1mm thickness (mind the usual PCB thickness is 1.6mm) other parts maybe had down to 0.5mm.
The result was about 7mOhms.
Again:
Initial: 13.4 mOhms, 100% current
normal: 10.8 mOhms, 111% current (11.3% increase)
ugly: 7mOhms, 138% current, (38.3% increase)
--> I see no way to reliably increase current capability to let´s say 150% by adding solder.
***
Not tested the increase in thermal dissipation.
It´s known that the solder stop increases thermal resistance. So omitting it will decrease the temperature rise.
But this is independent of the "added solder".
Klaus