Yes your method will work for a dirty contact but is only a temporary solution.
Using emery cloth (if the emery cloth is more than 1000 grit) and cleaner to clean the contacts I would say this would work for a couple weeks or months. The reason this method does not hold up is the emery cloth will remove the silver from the contacts. The way I clean them is to use a clean peice of cotton cloth with cleaner and close the contact and drag the cloth between them unit it comes out clean. This polishes the silver and does not remove it. Using emery cloth I have found causes the contacts to arc when they close and this causes a build up of carbon on the contact causing a the contact not to close completely.
At first there was a loose connection causing me to not have the 12volts. Then I found that the contacts/relay was closing and staying closed as long as the ignition was turned on, even with the relay light off. The contacts were not the problem. It was sending 12volts to the glow plugs but it was never turning them off until the ignition was turn off. This caused the glow plugs to burn out. They check open.
So I decided since I am senior electronics tech to check the components on the circuit board hoping it was a capacitor, transitor or rectifier but I found it was the Intergarted circuit or IC chip that is bad. Now since it is a special number chip that has no aftermarket replacement and can not get a replacement IC chip, the board is not repairable. Oh you could take a 255 timing chip and a NPN transistor, PNP transistor, 4 resistors and 2 capacitors and do the same thing but then you have to design the circuit using lot of math calculations to get the size of the components which could take days to get it right.
So the most economical method is to build a old style ciruit using exisiting relay without using electronics. This way I would only have to buy a pushbutton, glow plugs and take about an hour to build. Why they insist on making an electronic circuit like this for equiptment that is outside most of the time and when a simple old style repairable ciruit should be used could only be to make money off the repairs.
You do not have to use the existing relay you can buy small 30 amp 12 volt double pole relays for less then ten dollars. I replace them all the time on 4 wheelers.
and did not catch it.