myradiator still has a pin hole leak.

i need to fix it asap as it only leaks undewr load or when the car is stopped, but enough to fluctuate the temp by 20deg F.

radiator shop have had it 2 times this wekk and pressure test is ok but the pin hole still weeps under the above situations.

I was going to use a 25 to 40w soldering iron but dont know if it will heat the metal enough to bond the solder to the copper tank.

Is there a putty or similar i can put on it? bear in mind i have no access to any shops or workshops till monday.
im scared to take it on the freeway in 38deg C weathewr in case it boils over. its only 13psi pressure i think from memory so would silastic or similar do the trick or just a spot weld from an arc or similar?

thanks if you can help me.