Hi Karen,
You should have no issue with counter-enamel if the sides of your metal are clean .... and ONLY the side edges of the metal are coming in contact with the angled sides of the trivet.
1. Your problem could be the trivet size - test your metal before enameling to get the right fitting -
2. You may also be putting too much counter enamel on the back - rub your finger around the edges of the metal to remove excess enamel before sitting down on the trivet.
Let me know how that works out,
Trish