Can a 240V, 20A AC switch be used for 12V, 20A DC?

After setting up my DIY off grid solar power supply for my amateur radio shack, I had installed this two way switch to change over from battery to SMPS power supply. That was as a backup for cloudy days when the solar panels may not charge the battery well. This is a 240V, 20A switch meant for alternating current mains supply. That was because I could not locate a DC switch for 12V, 20A locally. But my ham radio friends warned me of the potential problem of using AC switches for DC. Now, what is the problem and what is the solution?

There is a possibility of arcing when the switch opens as DC gives a constant voltage. Hence the arc will be sustained for a longer period of time, generating more heat and may severely damage or weld the contacts together. Alternating current on the other hand naturally extinguishes the arc that forms when the switch opens because the voltage passes through zero multiple time every second. Switches for low-voltage DC applications may use special materials like gold plating to ensure a reliable low resistance connection. DC switches tend to be bulkier and more expensive. One solution suggested for me was that to avoid live switching to reduce the risk. That means I should switch off the radio before changing over from SMPS to battery. Radio uses only low power while receiving, which can increase significantly in transmit mode.

I was told by my automobile engineer friend that two way switches are being used in cars for changing between two types of horns. I am planning to visit an automobile spares’ shop soon to get hold of one of them. One snag which my friend mentioned is that they usually mention only 12V rating and not the current specification. Yet he pointed out that horns may use currents in the range of what my radio uses on transmit mode. Another option discussed elsewhere was using a car headlight relay which can handle about 80A at 12V. But that will be only an option for one way switching and not the two way option needed to change over between SMPS and battery.